P.M.
Well, he's obviously just not ready to accept the ongoing scheduling and requirements of gettin' trained. I've known quite a few kids who "trained" as young as 18 months, which basically meant their parents/caretakers got trained to take them every X minutes every day.
The kids cooperate as long as they can be coaxed, wheedled, bribed, ordered or bullied into it, but at some point it becomes clear to more self-possessed kids that they are not getting it. The nerve impulses, bladder control, bowel sensations, ability to actually deliberately trigger a void, and even more important, the maturity to make the whole thing their own, just take whatever time they take.
The good news is, when all that comes together, the kids get it quickly. They are interested for their own reasons, they are eager to make it work, and they don't need prizes, charts, or bribes. This time tends to arrive with fewer complications if the parents can just be patient and wait for it. You can't make a child pee or poop, and trying to make it happen tends to have the opposite effect. That's when parents call it regression, but really, the kids have just had it with the external controls.
Good luck, and I'll bet your son will train right on time if you let him.