If you could only read his little mind! There is no rhyme or reason as to the actions of a toddler. Most little ones just don't want to take the time to sit on the toilet. They are too busy for such nonsense!
Maybe so much attention has been made over this that he now actually fears the time he has to "sit." I already know you have tried making fun time out of potty time, so there's no way reading a fun book to take his mind off what he is doing will help. I am sure you have tried making a game out of "making a big one" in the potty and rewarding him when he does.
You sound like a very responsible parent and I wish I could offer a simple answer. The only hope you have is that he will just grow out of this stage and move onto another stage that will worry you once again. That's the thing about children. You want them to get out of one stage and then they just go into another stage you want them to grow out of! Good luck and good health!