I would check out a local cub scout/boy scout program. That's something that keeps the kids together for a longer period of time and does several different activities.
You could check out summer camp programs, but if they're for a week only that can be hard unless they have a friend to go with. If he's more introverted he'll just start to feel comfortable about the time he's having to come home. But if there's a summer program that runs for several weeks or each so he's can take it in, "digest it", go back get more. Extroverted kids have a better time at the week long camps, because they just willing jump in and get to know and do things. Being more introverted myself, it was harder not knowing anyone, especially when most kids are from a distance and after that week you may not have the opportunity to see that new friend. don't get me wrong, summer camps can be loads of fun, but if your goal is to get friendships that he can nuture through the rest of the year, then it's probably not as realistic. if he goes to that for the sake of just having fun, then he'll get more out of it. I loved summer camp. My most memorable time was when I went with a friend.
you could check about the local playhouse for kids activities.
or summer rec leagues for sports.
Nothing says he has to play those things and some kids aren't meant to be the "sports jock". Bottom line he has to be happy with who he is and what he likes. Confidence will come partly through the confidence he has in day to day tasks. If he feels like he can do things (and be recognized for it) then it will help him have the confidence to step into new territory and try new things.