Check if he has ,  or has had  a urine infection in the past. If so he needs I further investigations for it. It's quite common in boys and can continue into teens , but most stop by 8 . Don't give liquids or water after 6 pm.  Wake him a couple of hours after he sleeps or before he is likely to pass urine. Make him walk to the toilet and pass urine every night after waking up. This will condition him to wake up himself after some time.