I was so glad to see your actual question - I thought it was going to be keeping them dry at night as part of potty-training, and I thought I was going to have to tell everyone ONE MORE TIME that my oldest and youngest were not potty-trained until they were very nearly three years old! It's been a long time since I dealt with this issue, so my advice may be antiquated, but I'll tell you what I used to do. For one thing, I used cloth diapers. At one point, though, when I was having this issue, I would use a disposable with a cloth as a liner. Twenty (well, close to twenty) years ago, they sold disposable diaper liners, also - I used these at one point, too. Here's another idea - my mom was ill a couple years ago, and when she came home from the hospital was using Depends (but not anymore, yeah!) and was having trouble waking up at night to go to the bathroom. I used "Poise" pads inside the Depends. I wonder about using something like that - or my girls use night-time pads during their periods. Either one of these things inside his diaper may absorb enough extra to let him go through the night without making a mess or needing to wake you to be changed . . .
Good luck!