Hi K.,
Instead of a pull-up, you might want to try "training underwear". It's waterproof, but still breathable, and it goes on like underwear. And it's washable. So you could call it "special nighttime underwear" or something like that, and it would function like a pull-up, but it wouldn't LOOK like a pull-up, so your son might be okay with it.
I don't know if they sell them in stores, but there are plenty of places to get them on-line (mostly cloth diaper websites). Happy Heinys sells pocket trainers - you can see them at www.happyheinys.com), and I think Bummis sells them, too (www.bummis.com).
Just a thought - hope that helps! Feel free to e-mail me if you would like more information on trainers. Good luck!
J. :-)