It happens. There is really nothing hat helps. Limiting drinks just makes him not feel the urge to go, making them wake up every little bit does nothing either. They are asleep and not awake. It only makes you get up, adds to the laundry, etc....
We just deal with it. Buy pull ups for bigger kids and wait it out. You should consider if the child is constipated. The full insides presses on the bladder and can actually press the pee out. That is the only thing the urologist did when my oldest grandson went to see one. He didn't prescribe meds or anything. He said is was still acceptable for kids to wet until they were teenagers even.
I read today at the pediatrician that 1.5% of kids still wet the bed at 18 years old.