Hi. I don't think there is a "norm" in terms of time spent socializing outside of school. Truthfully, if your son doesn't complain about the lack of social opportunities, but doesn't seem depressed, I wouldn't worry about it too much.
If your does hope to attend college someday, it might be helpful for him to at least have some kind of club or leadership involvement to list on his applications, but even volunteer work can look good. What does your son like to do with his free time?
Some kids are actually more mature than their peers, and just don't care for the "mundane chit chat" and "one-upmanship" that a lot of their classmates might waste time talking about. (Stories of who has a crush on who, or boasting about underage drinking at a party, etc. are not for the kind of conversations most parents want their kids to engage in, anyway.)
You might find that when he's old enough to start driving, he becomes more inclined to "hang out" with some other kids. Maybe he's a "late bloomer" and will become more interested in socializing in college, with kids who share similar interests in the same major. For now, ongoing weekly participation with the church youth group seems like a reasonable thing to do.
To get other parents' opinions, you might like to join Greatschools High School Parents Group at community.greatschools.net/groups/11529