Hello,
I would keep him in diapers or those pull-ups that look like underwear but are basically a diaper/pull-up for night time so the child feels like a big kid and if he goes over to anyone's house for a sleep over when he gets older, he won't be embarrassed. My husband had this problem when he was younger and he eventually grew out of it when he got older (I can't remember how old he was). It didn't matter if he stopped drinking liquids as soon as his supper was over which was 3 hours or so before bedtime or not. And he would go to the bathroom before bed and still wake up wet in the morning.
My stepdaughter had this problem for a little while and she grew out of it within a year.
My neighbor kid also has this problem, and they put him on some sort of pill, but as soon as he stops taking it, he's back to wetting the bed, so they have opted to put him in pull-ups instead of putting meds in his system.
I personally wouldn't be worried about it. It's more embarrassing for the child, so I think the less you make of it, the better. And I don't see the harm in him being in diapers still - it's more comforting and worry free for both of you. He will grow out of it at some point. Mention it to his doctor at his next appointment just to make sure it's not something more, but this is a problem for a lot of children. I won't be surprised if my children have the same problem. I know how it effected my husband when he was younger - the less of a deal, the better for the child.
Take care!