Sorry, I don't have any experience with this problem only a suggestion (which you may have already tried). Try and overcome his aversion to the toilet by making it an 'attractive' place to be. An incentive chart might work on the back of the toilet door, every time he uses the toilet he gets a sticker and after say 10 stickers he gets a prize. Another thought,as gross as it may seem, is a container of lollies on a shelf in the toilet. These are things that he will have to actually go into the toilet to see and thus achieve. One more thought, have you spoken to your G.P. to eliminate any physical problems that might be hindering his progress, I have also heard of a cream that can be rubbed on the abdomen, used for children and also for incontinence in adults. Good luck!
Leanne, WA, 2 Boys 5 & 2