M.N.
I did not have this problem but I think that I would go back to step one with the potty training. Just start back at the beginning with constantly asking him if he needs to go potty and taking him to the potty. I would also work out a reward system, he gets a sticker every time he goes in the potty and maybe even after the end of the week if he has so many stickers then he gets to do something fun...go for ice cream with mommy...something that he would really look forward to but if he doesn't get the required number of stickers then he does not get the reward.
If he doesn't respond to this method then I personally would think that a little discipline may be in order. I only say that because he is/was potty trained and he knows to go in potty and not in his pants.
My daughter had been potty trained for a while when she started having "accidents" and I thought that she was having the accidents because she was getting so engrossed in whatever that she was doing that she was just waiting to long. I thought that up until the day that she had been playing in her play area, stood up, walked away from her toys, spread her legs and peed on the floor through her panties. I told her that that was not okay that pee pee goes in the potty not on the floor and I put her in her room for a time out. My daughter absolutely despises time out and needless to say we have never had an intentional "accident" again. So I know that a lot of people say do not discipline at all but in some cases I know for a fact that it works.
Good luck and lots of patience.
M. N.