It is really hard to give advice when I don't know all the facts. Why the change in schools and how far away from old school?
Activities in sports was that with in the school or in a community setting? If it was a community setting I would definetly get him back into it. Another thought is how about boys and girls club? Was there one at his old school or one used by both schools to help intergrate new and old friends.
The best thing we did when our 3 kids went from a montessori to public school was have them ride the bus! This way they made new friends, whom they saw every day and lived near by. That is how they actually made their current best friends. Even trying it one way, like home from school. This made setting up play dates easy too for the kids can get off at each other stops and then parents just pick up from the house. It helped us get to know the parents too!
My son was in TO little league for 6 years so he met alot of kids who didn't go to his school. But when he moved from elementary to middle school some of those old friends were there. Now in high school he sees alot more of these old friends. He is in football which started over the summer before he started high school, so that helped also with being familiar with the campus and new friends.
Just a suggestion about high school, they will have an orientation week in the summer, usually August. Make sure you sign up your son for it ASAP. Spots are limited and it is a great way for them to get familiar with the campus and meet fellow freshmen and older students too! It was huge in relieving the stress of a new campus and what to do etc.
Are you friends with the parents of his old friends? If so find out what they are into during the weekends, paintball, soccer, lecrosse, swimming, baseball, football and see if you can have him join. The key is mixing the old and the new so that he stays social and feels he has friends to choose from for activities etc.
Good Luck,
S. Chase