4 year old still peeing the bed

Mommy2Boys09

My son has been potty trained for about a year. but he has never gotten down the whole night time thing. I don't give him anything to drink several hours before bed, make him go to the potty when he starts getting sleepy, make sure he's warm when he's in bed and he will still pee! I have never trusted to just let him sleep in underwear because the majority of the time his bed time diaper is wet when he wakes up. I'm not sure what to do now, help please!