Oh, man -- Have I ever walked a mile in your shoes!
They do make "big kid" pull ups for exactly this purpose that are fabric-y on the outside that look like boxer briefs. Make sure your son knows exactly what to do if he does have an accident at night. He'll need to change out of the pull up in the morning and dispose of it. I always pack my son a trash bag or ziploc bag, in case there are any wet clothes. He can take his whole backpack into the bathroom to change, and just tuck any wet clothes away.
My son doesn't usually have accidents when he's sleeping at someone else's house or on vacation -- he just doesn't sleep as soundly when he's not in his own bed, and it's the sound sleeping that is causing the night accidents.
My son has one friend that he does sleepovers with pretty frequently. I know I can trust this mom, so I told her about it. That way she can make sure my son has privacy in the bathroom to change, etc. Fortunately, he's never had an accident at her house... knock on wood!