The interesting thing is that the OP stated the following......
"Now, I have been in contact with my local immigration. They have been told the facts of my particular case. I have been told that i may have to bring my wife to immigration with me on the day........I have the 400,000 Baht in my account, I have all the required paperwork.....he is legally my son...."
Interesting that immigration did not mention divorce and joint custody.