what time does he eat dinner, give him less to drink at dinner time and do not let him drink again that night. Go to wee wee before going to bed.
Buy a plastic sheet (shop which sells wheelchairs usu sells it),
DO NOT give him diapers or pull ups for bed. Wake him up once at midnight to wee wee, if he complains, then tell him there is a punishment if he wets his bed, and then carry it out. What is his favourite activity? eg. cut time to watch cartoon or time to play his favourite game.
Buy more underpants and shorts to change, buy one more spare bedsheet. It is thin, can dry overnight.