P.M.
Could be your son just sleeps better at your house. Perhaps he's more relaxed than at his dad's house. Perhaps there are street noises or other disturbance there that allow your son to wake and use the bathroom. There's not much of anything either you or your son can do about this inconvenience, so just shrug when your ex does his theorizing, and tell him he can google "bedwetting in young children" and get all the information the experts know. Which is basically that children will stay dry at night when their brains, bodies, and sleep cycles have matured enough.
I've recently seen (on this site) the suggestion that you actually encourage more liquids before bed. The bladder pressure might then build up strongly and quickly, giving a loud enough signal to the sleeping brain for the child to get up and use the bathroom. If you experiment with this, let other mamapedia parents know what results you get!