I too was a bedwetter. I understand his pain and bewilderment over sleepovers, etc. Way back then, ("then" being the 60's) they tried pills, waking me up, etc. and like your son, nothing worked. I remember having very vivid dreams where I had gotten out of the bed, walked down the stairs and went to the bathroom, but I would wake-up in a wet bed. My younger brother was also a bedwetter, so you can imagine my mother's days and she was a SAHM as well.
Strangely enough, when I turned 13 it all stopped. They never considered it to be a sleep disorder. Everyone thought I was too lazy to get up to go to the bathroom and didn't mind sleeping in a wet bed. Believe me, this was NOT the case!
My younger brother also had the same thing happen to him. By the time we were in 8th grade, it stopped. I'm not sure if sleep patterns change when you hit puberty or not, but they must because we both stopped.
I don't have any great advice, other than my mother always told everyone that I went to spend the night with about the bedwetting. Yes, it was embarrassing, but I didn't miss-out on sleepovers at least. Being a girl, the other little girls never made a big thing over it and I wasn't taunted at school, but then I went to a religious school and we were taught early on about bullying.
What I am getting at is that your son could end-up having a similar experience. There is hope. There is at least one famous person that I know of that was a bedwetter and that was Michael Landon (the father on Little House on the Prairie). He even did a made-for-tv-movie about the subject.
In the meantime, at home, at least, have you thought about trying Depends? I also saw a commercial recently tv that they are making these shorts for bedwetters that look fairly normal..just like shorts...so maybe sleepovers wouldn't be such a traumatic thing.
You are obviously very sensitive to his needs about this and although it makes a whole lot more work for you, please stay that way for his sake. I applaud you for that.
J.