I can't say I'm in the exact same boat, but I am frustrated that at age 3.5 my son wants to wear diapers at night. He doesn't even want to wear pull-ups or try to wear underwear.....
So I just wanted to post that you could maybe put 'puppy pads' under his sheet. They actually make a flat pad in the adult diaper section. At my store, they were cheaper than the puppy pads. If you did a couple layers of mattress, pad, sheet, pad, sheet, if you had a middle-of-the-night-accident you could just remove one sheet, one pad, and go back to bed. It may not help with training, but may help with your sanity at 2 am.