I think maybe the poor little guy has some issues that he needs help to sort out. Bed wetting at his age is often about emotional issues.
I think the best thing you can do for him is talk to a GP about getting him some counselling or seeing a child psychologist.
In the meantime maybe buy him some of the incontinence pants or pads from the chemist to wear to bed and take him to the loo in the night before you go to bed and in the night if you get up.