M.S.
He is ready for underwear all the way and if you are concerned about wetting the bed at night go and get some plastic pants that go over the underwear and have him were those at bedtime over his underwear.
Figure out when his schedule is for going to the bathroom. A few minutes before he normally wakes up go in and get him out of bed, walk him to the potty and have him go to the bathroom. This way you are training him to wake up and go to the bathroom himself. After a few nights of this he will be going on his own.
If you are worried about him holding it through the night then you need to cut his drinks off early enough that if he does have potty it is all done before he goes to bed.
Asking him if he has to go potty is a very normal thing to do for a good year or 2. Pretty soon he'll start going on his own, and then also there will be times that he will try to hold it as long as possible so he can play and you have to tell him to go potty.