A. You're doing too much as it is. Not too many grown people, let alone little kids, feel comfortable rolling into the middle of a crowd that already knows one another. When you try to insert your shy son into established groups, you're not making it easier for him -- you're making it harder.
This is a lesson I learned when I tried to "help" my Mari make friends. I'd wait for the sunniest day and look for the busiest neighborhood park and take her by the hand and say really excitedly, "Look, baby, they're sliding down the slide -- why don't you ask if you can slide with them, too?" And Mari would bury her face in my leg and tug me toward the side of the park farthest away from her potential "friends," where she'd proceed to have a blast playing in the sandbox by herself and get hyped about making it across the monkey bars on her own. Eventually, I realized she was having a perfectly good time solo, and that when she was ready she'd decide for herself if she wanted a playmate. And if she wanted to spend time with that child again, I'd make nice with the kid's mom and schedule a date, at which the two could either play together or ignore each other -- or some of both because that's what kids do. So let your child enjoy himself sans the helping hand. He'll find his own way.