Have you tried putting his BM in the toilet after he goes to let him know that is where it goes? Not sure if you're willing, but I skipped the pull-up stage with my 2 year old and went from diapers straight to panties. Even at night. We had a few accidents for the first 2-3 weeks, but now she will be 3 Sept 1st and hasn't had an accident in several weeks. Also, we tried everything as a reward...candy, stickers, chart, balloons, and finally a friend of mine suggested money. I never thought of it but I should have because my daughter LOVES money! So we took a jelly jar and made that the potty bank and every time she went she got some change. With her, we could give her pennies and she was just as happy as if it were a hundred dollar bill! lol But you can use quarters, dollars, whatever works for you. At the end of the week, we took her to the dollar store (again, she didn't care that it was a dollar toy, it was a NEW TOY!). Anyways, we took her and let her pay for it herself (of course, there wasn't enough to cover it in her bank, but she didn't know that). I think she really got a sense of pride doing it all herself. Well, I hope this helps and I wish you the best of luck!
~L.