Get a variety of sketching mediums (charcoal, ink, pastel, etc) and look into life drawing classes to get the sketching skills started. 

Sketchup is a free to use 3D modeling software many offices use professional to design small to medium size projects. It's also easier to learn and use than a pencil sharpener.

Before he gets too committed to architecture as a career, I suggest a little reading about the realities of it.

Architect?? by Roger Lewis is a good start.

Imo, sketching and physical models are better learning tools

Revit seems to be the bomb lately.

Most software at this point would be hardly different than playing a video game. I would instead encourage you to expose him to good architecture. Visit cities. Ask him to observe more. Draw more. Prowling around the architecture section of your library or bookstore wouldn't hurt either.

Sketchup is probably a very good start. It's definitely great to encourage his interests. I know I wouldn't be where I am today if it wasn't for my parents being supportive of my interests when I was about 9 years old. 

Perhaps you could also encourage him to undertake his own small project, like design a shelving unit, dresser or other piece of furniture for his room. He will learn how to think in a human scale, consider construction and fabrication methods and save you a trip to Ikea. You can hunt through used building supply stores like Re-store to save money.

I'd also suggest, especially in my case, be open to the fact he might take a different path in the direction of architecture. He doesn't have to be a licensed architect to really pursue his design related dreams as you'll see in some of the topics posted here.

Taking a shop class is a good idea to learn about making things and developing craftsmanship. That's what I did at age 13 and where I founded some of the best basic skills. 

1. I second the Roger Lewis book. Geezereffect, great suggestion.

2. Steer him towards Blender. It's a free 3d program. Get him interested in game level design. Going to architecture school with 3D modeling skills means you can slack off and ride on the coat-tails of your graphic productions. That's a huge leg up, especially for a motivated kid.

If you're near a college with an architecture program, look and see if any have an architecture summer camp program.  He may be a little young yet, but once he gets to high school, he'll probably be eligible.  The camps I've seen are typically a week long and it's a mini studio where they give the kids a few simple design prompts (like a chair or bus stop) and let them observe, sketch out ideas, make models, and have a critique.  It's laid back, but they get to see the process of design at a very basic level.

Make sure you're both aware of the dismal pay before he gets too infatuated.

No need to get any software. The best architects I know (and the best in the world) rely on a good pen or pencil and sketchbook more than any software.

Get him some fine art and perspective drawing classes and some drawing tools he likes. 

You could also get him some model-making tools - balsa wood sheets and sticks, X-acto knife, straight edge, glue. 

For that matter I highly recommend Lego. 

forget software - learn hand drawing/art/craft skills and "making" skills.  He'll have many years ahead of him sitting in front of a computer to apply those skills. 

The other really important skill set he'll need is writing and speaking persuasively.

I second all of the above. Also, you ARE a great mom! Good on you for figuring this out :-) architecture is a fun and rewarding profession. I am a lot happier in my job than most folks I know.

A few additional things:
--if you do get sketchup, look for the free version. There's no need to get the paid version, although they sometimes kind of bury the free version on the site so pay attention.
--not sure where you are, but in my city (Chicago) there is an after school activities program called Afterschool Matters and one of the little courses they do is a little architecture for kids course. They do it at elementary level and the program looks really fun. You might see if there's something like that in your town -- it might even just be a "design workshop for kids" of some sort, offered on a Saturday--I've seen this kind of thing frequently at art museums, if you have one near you.
--relatedly, assuming you are already taking him around to see house museums / other building museums on the weekends -- talk to their programming director, they will sometimes hold little activities and public engagement events you could sign him up for.
--furthermore, don't just take him to fancy museum houses--if there are any house developments under construction near you, take him to see the model homes. My parents did that with me and you can always get a brochure with the house plans up front, and I would carry that through the houses with me, marking up things I liked or didn't like with a red pen brought specially for that purpose. It taught me how to read plans and relate them to 3D spaces, and also taught me what I liked and didn't like. If you only have house museums available near you, you can do the same thing by looking up the published plan and printing it out ahead of time, so he has something to look at and make notes on while he's there. Once he's looked at a few plans, you could take him to a house without one and see if he can test himself by drawing the plan himself as he walks through.
-- other good books for graphic learning and drawing are the Francis Ching books. Very easy to read and a great primer on architectural terms. There's also a few good Visual Dictionaried of architecture that will teach him to identify different fun parts of buildings -- kids always like it when you can turn things into a hunt / game of some kind. You could keep it in the car and he could cross off each term as he finds it "in the wild", like architectural bingo.

Thank you everyone for all your comments. It has always meant so much to me to give him the chance to explore this design world ever since he was 6 years old and told me he wanted to one day build me a house. We had a local cinema/ shopping complex built nearby and he insisted we visit regularly to see the progress of the build. I have always kept in mind that he may not keep to the idea of being an architect, however it is something that keeps coming up many times as an interest to him. I think I have a much better idea of how to help him understand more about the architecture world and get his creative juices flowing. Much appreciated!

Are you guys kidding the OP? With all the posts about dismal pay and the crappy lifestyle associated with architecture, why are there such less posts about how unsustainable this business is, and especially after 10 years wherein the modes of design and production keep getting more and more automated.

To your son, Id advise that there are many more professions where the architectural skillset can be used to a greater benefit both for oneself and society. Do your research!

Not all of us feel that way same old. I earn good pay for challenging and varied work for good bosses and with good colleagues. My cousin is an architect in London and the financial picture looks still better over there due to lack of significant school debt. Not everyone sees the world the same way you do. I don't make as much as a doctor or a lawyer but I would hate their job. Meanwhile I'm still doing better than the vast majority of people in the USA, and planet-wide I'm downright absurdly wealthy. For someone whose grandparents were refugees, that's pretty damn good. There aren't that many other professions that pay as well for such easy (i.e. Not back-breaking) and interesting work. If the OP keeps her eyes open to the finances and guides her son accordingly I'm sure he'll be fine. I know you're just trying to make sure ppl make realistic decisions without rosy glasses, and that's admirable, but it would be equally misleading to say the profession universally sucks and everyone is miserable, and we hear a very vocal minority overly frequently on that subject I think. The OP is obviously a reasonable, thoughtful type, I'm sure she'd steer her son clear of any terrible financial decisions.

sameold, when someone is 12, you encourage his or her interests whether they be in music, art, science, math, writing, etc. Can you imagine telling a 12-year-old not to study music because there is no future in it? Discussing the financial outlook of a profession is something you can save until he or she is looking to go to college.

Architecture isnt much like Grand Designs tbh which does spice it up a lot - its mostly dull CAD drawing and dealing with annoying consultants and crude contractors! I cant recommend it as a career personally unless he is very practical but also he needs to be artistic and a strong communicator/leader to get anywhere otherwise he may get stuck in a mundane position doing CAD all his life.

Has he considered film design / architectural visualisation / computer games design? At 12 I would be opening up his eyes to possibilities not encouraging him down a certain route just yet.

Architecture can be a good career but its a difficult one as well.

Anyone can learn to fiddle around with softwares.  Get him interested in other areas.. Experience culture, science, arts, history, writing and everything that seems unrelated to architecture but will make him a better designer and critical thinker.

as a high schooler, my folks encouraged me to build something in the house...i did a raised floor area with a couch...the back side had a bench...all decent (but not great) veneer plywood

it was pretty awful design-wise...it's still there and a sad reminder of my earnest but uneducated design decisions