I sympathise. DD1 is now 3.11 and has been potty trained since 22 months. She was fine at first, but a spell of constipation really set her back. She has has accidents on and off, but it has gradually got worse.
I recently took her to the GP and she has taken it really seriously. I've tried all the reward charts, favourite pants, getting cross.... you name it. GP suggested further monitoring then referral to a paediatrician.
Anyway, over christmas we spent 4 days away and she was dry throughout! As soon as we were back she reverted back to wetting once/twice a day. We then felt that there was some element of consciousness about what she was doing and felt able to be a bit more direct with her.
Both DH and I had a chat with her where we pointed out that she had managed to stay dry at Aunty's house, so she can choose to do the same at home. We talked about her showing how she can be a "big girl" and make her own way to the toilet. It seems to have worked .
I'm not saying the same thing works for every child, but from what you say, your DS sounds like he may stay dry when away from home/CM. I'm sure you have tried this approach, but I thought I would share it, just in case it helps and also to let you know it can come to an end.
Sorry, I've rambled. Anyway good luck.