I would suggest you contact Chalermpol Intha intha@rcn.com.  He is an Architect  in  the Boston, Massachusetts area but he might have a contact in Thailand.  He has been involved with some United Nations work in the past.  He goes by Charles  Intha.

I just got back from a three week trip there.  Although the people, culture and food are fabulous, outside of the temples, the expensive resorts, and a handful of stuff in Bangkok, architecture (and building codes) are pretty much non -existent.   So just be prepared as a volunteer you would probably be doing mostly stuff like  building out of palm leaves and bamboo in areas that were trashed by the recent floods, or working to help an orphanage build a school out of concrete blocks and tin. Don't forget the threshold- it keeps the evil spirits out....