It is easy for me to say "yes, you are worrying too much" but having survived raising three kids through that age (and one more to go) I'm not sure I wouldn't be a little worried, too.  Here are some things I would be considering if in this same position:
 
1. Is he happy?  This is often hard to tell at this age, but there are cues to pick up on...a spring in his step, the occasional whistling, self-motivated curiosity about things, being positive about the future, sleeping well, eating well.
 
2. Is the situation stable?  With just one connection to the social affairs of the school, that seems pretty fragile.  If that one friend moves away, gets interested in other things, gets a girl friend, or just grows in a different direction from your son, that would be sad.
 
3. Is the one friend a good kid?  Better to have one good friend than a gaggle of goof-offs or worse.  At that age, my older daughter didn't have many friends but they were all good kids.  She was selective.
 
4. From friends come new friends.  When I look at my kids' friends and how they all met, I see that a lot of them were originally two degrees of separation acquaintances that turned into friends through group activities.  At 13, there are some things that bring kids together.  My boys were (sigh, still are) big AD&D fans and a group gathers here almost every weekend. Now it is a fully-connected group.
 
5. Related, what about organizing one of those weekend things?  Your son and his buddy could invite a couple of others over for something...a game of hoops at a local park, Guitar Hero (for all the bad rap GH gets, it does bring the kids together), some FRP board game.  Start small.  Be prepared for it to go either way, but keep trying.  If it succeeds, you may lose your kitchen every Saturday night.
 
6. Talk to the basketball and soccer coaches, mention that your son doesn't seem to have many close friends and if he (the coach) sees any yellow flags.  Often the coaches see some things in kids than parents do...they don't have the mental inertia of "little Jimmy".  They just see the kids as they are today.
 
If more ideas come up, I'll follow-up.

I have the same issues with my 15 year old. Unfortunately, he is not a straight A student, but the rest of what you describe applies except my son has Asperger's Syndrome.
 
I agree with what the other responder stated about it being a "fragile" situation to have only one friend. What if that friend moves? It has happened to our son 3 times already. Fortunately, we live in the technological age of cell phones and XBOX live. He has been able to keep in contact with 2 of his friends this way, but other than the little bit of socializing (if you can call it that) that he gets during the occasional phone call or gaming session or during a cross country meet, he goes nowhere and hangs out with no one.
 
I have often talked to him about the people in his class, on his team, those with whom he eats lunch every day. I have encouraged him to invite them over to watch a movie or play XBOX or go out to the movie theater, but he really has no interest. I think the social awkwardness attributed to the AS plays a large role in this, but also, as the other responder reminded - He seems perfectly happy! He really doesn't crave going to hang out with friends. He enjoys being at home with his video games, books and pet rats. He SMILES a lot. He LAUGHS a lot.
 
The hormone surge of puberty was a very trying time for us when dealing with an AS teen, but he seems to have regained most of his self control, and I have seen the pictures he takes at his cross country meets with his camera phone. Nice, funny, cute, happy pictures are what I am seeing. And I am seeing mostly the same faces each time. Perhaps he has gained a "few" friends now, and perhaps the amount of time he spends with them during the meets and practices is enough for him. I guess we will see how it all plays out when this sport's season ends.
 
I am thinking about trying what was suggested previous to my post. Halloween is coming soon, as is the end of the cross country season. I may try to host a Halloween party of sorts, but then there are the questions of who to invite and how many and will this be something in which my son is even remotely willing to participate?  So many questions, so few answers!

My son had socializing issues in the beginning of 6th grade, he is now in the 8th and has a lot of friends.  I took him to a therapist last year because he became very insecure, and even had some issues with being angry because of his insecurities.  If you feel you want to take him to see someone, try a therapist first and more forward from there.  Sometimes they just need someone else to talk to other than mom or dad.  I'm sure it is just the age, middle school can be difficult for a lot of kids.  I hope this helps and good luck.

Have you ever thought about the possibility that he is so successful at school BECAUSE he isn't hanging out so much with peers? From your description, he sounds like a very "together" kid and one that doesn't necessarily require a lot of people around him. He sounds very content and introspective. My oldest child (of six) is very much the same way. She finds people her age (15 going on 16) annoying. She doesn't like the gossip, the drama or the fact that kids her age are (for the most part) not very academically driven. She can't find anyone who is interested in the same music or literature and so she finds joy in those things on her own. I could have let that worry me, but I let her find friendship in her own time and in her own way. She has recently found a really good friend that she enjoys being with and that is enough for her. She is a home educated student and also attends a local community college and interacts very well with students and professors and not surprisingly she is actually a little more mature than many of the students who have already graduated high school.
 
Your son will be fine. To pursue help for a child who is healthy and happy might turn a good situation into a bad one. You just might make him feel like he has to hang out with the wrong crowd and neglect his grades just to "fit in" if he ends up feeling as though there is something "wrong" with him due to the psychotherapy. Give him time and space to make friends and allow him to be "him". He sounds like he is doing really well....just not necessarily doing what you would do. And I would also suggest that you enjoy the time that he wants to hang out with you and your husband-- one day you'll look back and wish he would hang out with you again!
 
Monique Zarcone
M.A. of Ed in Teaching and Learning