My son is 6 and still has that problem. His doctor said it should stop on its own. if it does continue after the age of 8 then to consider getting him checked. so right now we are just waiting till he turns 8 and maybe he should stop by then. One thing is don't get mad at him, make him feel bad.
My friends son had this problem. Try to stop a drinks for 2hrs before his bedtime and invest in those pull ups that they have at the store for kids who have this problem. When he is in such a deep sleep that trigger in the brain that wkes us to tell us we have to go is not sounding off. This is what the doc told her and what to do and it did make a difference. Hope this helps
My son wet the bed until he was five but it was only at night. He wouldn't even wake up after he wet the bed. His pediatrician said that he would out grow it and also that it could be heriditary. He gave me some broshures on different gadgets you could buy to decrease the wetting. I never purchased them, he eventually stopped 4 months later.
It could be a medical condition if he also has this problem during the day. You should talk to his Pediatrician to rule this out.