He's not an only child, but my eldest homeschools. There are still isolation issues with kids who have sibs.
One approach we use is to send him to an after-school gym program at the local public middle school three days per week. Something like a hundred or more kids go, and so he gets to experience the large group thing. Plus, it's basically a rigorous PE class. They play games (Capture the Flag, Floor hockey, Dodgeball, Basketball, etc.), and it's really good for him. My impression is that he spends the entire time moving.
