I had the same problem with my girl when she was 10. I also have see this kind of questions pop here and there. The more I think about it it really seems that our "no so kids anymore" go through another change between 10-13.
Bff last year now have different interest, some now are datting, some have more technology acces (phone, face book, twiter, etc) some are staying the way the use to be a year before.
Is normal many friend ships get blow away with all this changes then add hormones, yikes!
To add to this, sometimes as a parent don't see that our kids also avoid this or other kids. back in Florida our house was the go house to all kids in my block, but my daughter would NEVER go to the other kids house. I always will tell her to go but she wouldn't. Eventually some of the kids stop coming and I understand. Friendships must be mutual.
I had a talk to her.
This year she had less friends but better ones, well, no better but they have more things in commun.
However, during the vacations, she never call no one, I ask her why she said she didn't have nothing to say...She just came back from camping, learning archery, hiking and canoeing! I made her call and she was super happy on the phone for an hour!
Talk to kids is like walk in thing ice, you never know when you are going to fall to a freezing call water!
Maybe your son needs new friends, or maybe he needs to step up and look for them.
If they don't have nothing in commun a new activity will bring new friends and a improve in his self-esteem.
He can improve an already know activity or jump in to a new one.
My daughter never camp in the forest, never canoe, she was a book worm and a artist and most of her friends were too
But even if she came back with tons of blacks and blues and tons of mosquitoes bites, she now open her self to new friends.,