M.S.
do you have cool underwear with thomas the train or some other character on them? some kids are afraid to get Thomas dirty.... you might try that...
how does he feel about diapers? if they are a "baby" thing to him, maybe it would be motivation to put him in a diaper after he has an accident, and tell him he has to wear it until next time he goes in the potty. if he doesn't care that probably won't help......... but it's an idea......
my daughter has a hard time pushing it out on the toilet-- she prefers standing up when she poops... maybe a different toilet seat, or having him put his feet up on the toilet so he's squatting?
good luck! don't worry, he won't be doing this when he's in high school!