We used to get our son up in the night in winter to go to the toilet, but I think just the cold weather makes it worse sometimes (although not sure exactly why)

Make sure he goes to toilet before bed and don’t offer drinks at night before bed time

Anything I say you’ve probably already tried, it could just be the cold weather, I know I sleep heavier when it’s cold. Don’t allow drinks for a coupke if hours before bedtime and make sure they go to the toilet before going to bed. My dad used to wake me when he went to bed to take me to the toilet, bit cruel but nicer then wetting the bed