I'm 16 and I wet the bed. I've actually been dealing with this since I was about your son's age. For a while I was always afraid that someone would notice and so my mom would pick me up before everyone went to sleep. We said it was because I got home sick. As I got a little older (ie like a year) I started to stay. Depending what kinds of diapers he uses he might be able to slip it on while in his sleeping bag. If not I always slip it between my pajama pants and my hip and take a trip to the bathroom just before bed to put it on. If he is a somewhat early riser like me he can get up before everyone else and change before they wake up. If not (had this experience too) the first step is a trip to the bathroom the moment he wakes up. If he plans to put the diaper in a plastic bag in the morning which I suggest, he should try to stick it in the bathroom before going to sleep. Somewhere where only he will know and where he can remember. It would also be helpful if he gets too nervous to do that (as I sometimes do myself) or if the diapers ever leak, to sew a waterproof pad thing in his sleeping bag. This ensures that he won't ruin anything in his friend's house. Make sure his friend's parents know he wets the bed beforehand and let them know what his plans are to take care of it during the sleep. Also let them know his feelings on the subject. Encourage him to put his sleeping bag near a bathroom if possible. He doesn't want to have to walk over people or around people in the morning if he's an early riser. I hope I'm not forgetting anything.
I've had a lot of experience with bed wetting at sleepovers over the years. Sadly, it requires a little sneakiness, but this is how I've coped and it's worked pretty well for me. Hate to hear about other kids going through this. It's not easy. :(
Hope he has a nice time at his party and remind him to have a good time in light of it because it is a sleepover after all and worrying all the time is just no fun :)