Hi L.,
I know this can be frustrating for both the parent and the child. You are totally right, when his body is ready, he'll be able to control the bedwetting. Sometimes it just takes the bladder longer in some kids to mature. There's some evidence that this may be a genetic thing...if a parent hsd this problem as a child, then a child may have this problem as well.
About the sleepovers, I have two thoughts. First of all, would it possible for your son to put the pull up on BEFORE the kids arrive? This way, maybe the kids won't notice that he's wearing one. I think there is a brand of pullups that is designed specifically for the older child, and is designed to not be so obvious when worn. I don't remember the brand, but I want to say it's Goodnites, or something like that.
Second, you may have done this already, but maybe you could ask your son for ideas about what could be done to help him with this problem. Oftentimes, kids have some great ideas for how to handle embarrassing problems like this. You said that you'd like to help him get more confidence in controlling his body...and asking for his ideas and then following through is step in the right direction. It's his body after all, and he should be included in solving problems.
I think you're headed in the right direction...the more pressure you put on him, the harder it is for him to feel in control. It's great that you're backing off and not putting pressure on him. Good job mom!
One other thought, you may want to chat with is pediatrician about this. His doctor may have other ideas about how to handle this.
Also, this will eventually pass, so remember to breathe, and do your best in supporting your son. You're doing a great job so far, so kudos to you!
M.