Well, you could just get the thick, cotton, absorbent training pants (Gerber makes some) and put a waterproof pad on his mattress, then a sheet, then a pad, then a sheet to make it easy if it does happen.
And while he sounds like he's well on the way, sometimes kids don't stary dry at night til a year or so later...sometimes 5, 6, 7 even...it's not about "training" at night--it's physical development that may or may not be there yet.
I used Pull ups until we had 2 dry weeks in a row & then he was good to go with reg undies. Good luck!