He doesn't sound "weird" but unique, he'll find his place and if he is happy, he's fine
Does he chat online to the people he does the horse stuff with? That, combined with talking and dealing with people at school could just be enough socialising for him
I kinda wonder what you were like growing up? I was a bit rebellious, fairly popular, not top of the scale, but friends with those who were, physically ,mature outgoing, sociable, confident and still am really
My daughter is quiet, insular, totally utterly a rule follower, not popular, not unpopular but so quiet she glides right under the radar
It was hard, it is hard, for me to accept that her school holidays so far consist of going out with me for walks, to the shops, out to eat, or out with me and a friend and daughter that I organised. That she and her particular group, don't just meet randomly, they're all similar, never any spur of the moment meets, they don't just meet at the beach, be there or be square . . . Yet at work, colleagues children have barely been home, camping in gardens, teen barbies, parties, beach day after beach day, just meeting for a wander round town . . . To ME her life is weird and antisocial. To her, it's happy!!
Is he happy? If yes, keep talking to him, make sure there's no dodgy meets with gamblers and listen to him . . . And buy him a couple of trendy tops 