This sounds so baffling, M.. It's also an amazingly common situation. If you were to ask your son what his understanding of the situation is, he might reveal the best path forward from here. You might try making a quiet time together, with your focus on your son and some quiet activity like coloring, and make an observation:
"Joey, I was so happy for you that you were using the potty so you wouldn't have to use diapers any more. Now I notice that you're having accidents every day. And we all have to stop whatever we're doing to get you cleaned up. I notice that takes even longer than if you just use the potty, doesn't it?" Try to keep the tone purely factual, reporting your feelings and the new pattern that's emerging. Then be silent for awhile, and go on coloring, and see what he has to say.
He may tell you that it's too hard to remember all the time, which would indicate he's not quite ready to accept the full commitment. If this is the case, I would cheerfully offer to put him back in diapers. He will be ready, and want to potty like a big boy, soon, and with no pressure to train for someone else's sake, he will train for his own sake. And when that happens, the training will be very fast and self-directed. Just like when he was ready to walk, he probably went from a couple of awkward steps to a determined walk in a matter of days.
He may tell you other things, as well. Perhaps he'd like you to remind him more often. Or less often; perhaps he's just feeling pressured and a bit resentful. If you're able to find out what's actually happening from his own viewpoint, you will have a clearer sense of how to proceed.
I strongly recommend the wonderful and practical book titled How to Talk So Kids Will Listen, and Listen So Kids Will Talk. I've used this approach with my Grandboy 4.5 to very good effect. He's amazingly cooperative and sensitive to other people's needs once he's convinced that his own emotional needs have been taken into account.