I don't know that I can help, but we're having the same problem so I can commiserate! Mine is 3yr 3mo and was not quite as completely trained as yours, but hadn't had a poop accident in several months. He was doing ok with pee, but inconsistent. Now, for the past week, he never tells me he has to pee, just goes right in his underwear and then tells me. He has pooped his pants a couple times also. As long as I tell him to go potty every couple hours, the accidents are minimal, but if not, he's basically using his underwear as a toilet all day long. He is only disturbed by it when he poops his pants. If we go out somewhere, he never has an accident, although I usually have him go before we leave. But fairly often we'll be out and tell me he needs to go and be able to hold it for at least 10 min if needed. We also have the problem where we ask him if he has to go or thinks he could even do a tiny bit of pee and he says no, and proceeds to pee his pants 5 min later. Today I asked him if there was a problem or something, because it's just yucky to pee/poop in your clothes and he just told me "I'll do all my pee and poop in the potty when I'm all grown up". Hmmm....
I just don't get it, he is developing normally in all other aspects. I don't know if I should just keep reminding him when to go, or continue to let him go through 4+ pairs of underwear and pants each day (uggh...more laundry!).