Why is my son so antisocial?

Because he has been home schooled, most children make their friends at school, he will now be an outsider from the neighborhood children as they already have regular acquaintances with each other from school and will have more in common. You must have realized this would happen when you decided to home school him and at least got him in something where he could have made friends with people his own age. Teenage years are difficult enough but he probably doesn't know how to socialize with people his own age.