This is just a thought, but ... when he was first getting trained, you were "constantly" reminding, or offering him to use the bathroom, right? Have you stopped doing that? It's possible that he was much more dependent on your prompts than you realized. At school, particularly in the pre-school and kindergarten levels, but sometimes into the lower elementary grades, there are bathroom breaks. He might not have to pay much, if any attention to when he "has" to go at school. The other issue is, he might be very involved with things (TV, video games, legos, whatever), and his focus is so intent on what he's doing at home, that he can't/doesn't notice what's going on in his body. You may need to make sure he takes potty breaks every hour or two, since he isn't doing it on his own. The more involved he is in his own activities, the less likely he may be to monitor his need for bathroom breaks.