M.J.
I just wanted you to know you're not alone. Boys, in particular, can be so difficult to train completely. Our son was trained at three, but still has accidents at five. He'll do fine for months, then have a week of accidents every day, with nothing in particular causing it. I think he is just too busy enjoying life and using the potty is an annoyance so he tries to skip it.
We've done it all from small rewards to big rewards, to cheering to punishments, none of which made one bit of difference. Now, we're just nonchalant about it. It happens.
I have heard that peer pressure can help a lot. We're hoping the move to kindergarten next month puts an end to the accidents once and for all. Perhaps preschool kids will at some point put pressure on your son to stay dry?
Good luck!
M.