Good Morning J.!!
I have 2 suggestions for you. 1. Depending on your child's body type, it might help keep him dry to go to a night time pull-up. (Huggies makes Good Nights that are really absorbant). But if your son is a littler guy, these might be to big and you'll still have the leak problems.
The other option is to purchase something called "diaper liners" or "diaper doublers". I used to find them at Meijer.. then they discontinued carrying them.. then I found them at Rite-Aid. Basically they are like a large maxi pad that you put inside the diaper for added absorbancy. If you can't find them at Rite-Aid or maybe CVS.. you might want to try using an adult incontenance pad (like Poise) and insert that in his diaper for the night.
Hope you found this helpful! Good Luck!!
Peace!
B.