I've also posted shared your question on our Facebook and Twitter pages so I hope we'll get some responses there as well.
Do you have any further questions regarding relocating to Thailand?
So the move to Thailand will be a pretty big change for them.
Did you find the article useful? Have you joined any expat forums and connected with others living in Chang Mai?
The prospect of moving overseas can be both exciting and daunting for children. I wouldn't say there is ever a best time. When they are young, it will obviously have less impact on their education but then when they are older they'll be able to appreciate the experience more.
Ultimatley, it will be a decision that you'll have to make together with your son. Obviously, his education will be a prioraty and you'll have to look at international schooling options which'll offer the least distruption to his studies. International school fees in Thailand are very high and your best choice of options will be in Bangkok.
We have an excellent article here that I think you'll find useful : Preparing your kids for living abroad.
Have either of your kids every lived abroad before? Are they used to travelling and experiencing new cultures?
I have just read your post and am wondering what plans did you make or what educational prospects were available for your eldest child as I am in a similar situation.