This message might come slightly late, for which I apologise. If you manage to read it then I hope it reassures you in some small way.
Now, at the age of 21 my life couldn't more more different to what it was four or five years ago. Indeed for many of my formative years I had no friends at all. It's difficult to boil my former friendlessness down to one single issue. I was always a quieter individual, perhaps a bit of an eccentric and oddball. This attracted the attention of bullies and my years in primary school were extremely difficult ones. I started secondary school hoping that I could meet new people and make friends, in fact I can remember being preoccupied by this in the summer holiday prior to starting. In hindsight I think I lacked adequate social skills. In the first few weeks of the school year I made a huge effort to make friends, but it fell short and certain members of my peer group were unpleasant to me. Faced by this adversity I decided that it would be easier to retreat and keep myself to myself. From thereon in, my experience in School was one of survival. I spoke to a few people occasionally but most of my break times were spent standing alone in the playground. My lack of social interaction in school was accompanied by a complete lack of it outside. Like your son I didn't play any sports, and because I didn't have any friends my weekends and evenings were extremely lonesome.
From what I can remember, I didn't experience loneliness keenly until I was about 15 or 16. I had my moments as all teenagers do (puberty is a very difficult time!), I was tired quite a lot and school was pretty frustrating. I think your Sons behaviour might be linked to this as opposed to his friendlessness. At any rate, I was initially shielded from the reality of my situation. Until the end of year 9 I lived some way away from my school (and where everyone else lived), because of this I was sheltered and got on with life so to speak. During that Summer we moved to an address much closer to the school, in the neighbourhood that many of my classed mates lived in too. Seeing them around, going to parties together and socialising highlighted my isolation and I felt extremely lonely much of the time. Hearing them discuss parties and outings that I simply wasn't invited on genuinely hurt. I wanted to break free of the social situation that I existed in, but there was little that I could do. I had few social skills and was regarded as the guy 'nobody really bothered with' by most of my classmates.
Leaving school and starting sixth form presented me with a new opportunity to make friends and meet new people. Although this started somewhat haltingly, by the end of the first year my social life had improved substantially. I had made friends and was actually going out with people to parties and the cinema and so on. Through the Second year, I continued to socialise with people. I think this experience helped me nurture social skills meaning that when I left for university I was able to meet people and immerse myself in a normal social life. Three years later, I'm on the verge of graduating (don't really know what will happen next...) with a girlfriend and great circle of friends.
Things might well work out for your son as they did for me. He is only 12, and in the next six years will change beyond recognition as he grows into a young man. If you support him no matter what, and act as a source of support and advice I think he'll be fine.