My son has no interest in playing baseball. He is 8 years old. My husband insists that if he doesn't play he won't develop and join in other activities. I am torn and I don't know what to do.
You're both right. Your son's preferences should be respected -- he shouldn't be forced to do something he really doesn't want to do, outside of the basics -- homework, chores, treating others nicely, and following safety rules. Some kids love sports, some kids love baseball, and other kids don't.
But your husband is right when he points out the importance of encouraging your son to join in activities and learn to contribute to a team effort. There are huge benefits associated with team activities.
Also, some kids just "hang back" more -- their personality is a bit shy, or they're overwhelmed with a lot of noise and activity. That's OK. You can help him select something that matches HIS personality.
So, how to balance both sides of the issue? Start with your son. Explain that he needs to join SOMEthing -- create a list beforehand of sports activities and clubs that both you and your husband agree would be good possibilities, and see what interests your son. Perhaps he's just not a team sports guy, so maybe karate would be better for him. Other kids who don't like baseball sometimes like swimming or tennis. Or if he's really not into sports, check out the other clubs available --- math clubs, robotics, photography -- the sky's the limit.
But once he agrees to follow through on a season, a set of lessons, or an activity, he needs to complete the whole series. Make sure he knows the rules beforehand, and give him lots of support for the steps he's taking along the way. That way he'll eventually learn the benefits of sticking with something in order to benefit from it in the long term.