We have that issue with our 7 year old!!!! She's obviously been potty trained for a few years and she will have periods where she will wet herself because she waits too long.
We've taken her to the doctor, punished etc. What we've found is when there's some kind of upsetting issue at her mom's she starts the peeing issue. Once we figured that out, we keep in close communication with her about what's going on at her mom's and head off any issues.
So yes, it could be jealousy. Ours has a 2-year old sister at mom's who gets a lot of attention, and often the issues center around being jealous of her sister.
If he starts with the peeing, I'd have a talk with him. Make sure you do a lot of listening and don't make him feel bad for being jealous. In fact, him just admitting he's jealous is often enough. Once our girl gets out what's bothering her, everything clears up immediately!
(We still do punish though, usually she has to clean up her pee and lose TV. We punish for not being responsible, not peeing. Ours is old enough to take responsibility for her body.)
Good luck!