By far the most important thing, I would suggest, is to get your lad less distressed and more happy asap.
By all means get help from the doctor if it looks like its necessary.
But, sorry to keep banging on about this but it is the main point - if you just spend all of your time contemplating this deep black hole, with or without medical professional binoculars, you will make it get bigger. You and your son will increasingly convince each other that this horribleness has come to stay and make it harder to shake off. The reality is, it's probably short lived, the world turns and moves on, things change and one day you stop and realise ,"hmm, long time since we thought about that problem now, all he seems to talk about is...(insert pleasant activity that your son is now keen on and occupies his thoughts.) That would be the best outcome, wouldn't it?
School is a problem right now and the kids outside (same kids?) likewise. He needs more strings to his bow. School isn't the world and neither are the nasty football kids.
He needs a bigger sample of humanity in different settings to get some different and better behaviour models and feedback from than his peers at school are providing at the moment. And some FUN. And from the sound of it, so do you. Go and do something with him. Thats tiring and active and fun. That also involves other people and kids and is supervised and structured as Adoptmama says. Spend time being active and having good clean fun and it also starts to invade the rest of your life. You get optimistic and positive. And you get the sort of friendships and behaviour models you want - friendly active people who like to learn, develop and have fun; not people whose focus is teasing, name calling, setting people up and generally being an . Running clubs, karate clubs (its not just about fighting, see their websites), walking groups - sign up for a challenge and its best of all if you can model it for him by doing it too and supporting him while he's there.
If you go out and have some fun together with other people he will be happier. And more optimistic. Thereby more resilient, less touchy. And have other models of more positive behaviour that he can take back to school, especially if you help him make the connections. That's more than half the battle won. Hard truth is that your son is learning that those nasty kinds and even groups of people are out there and sometimes you just don't feel like you fit in. He will be meeting this situation whatever age he is, repeatedly, all his life, like we all do eg. at work. His lesson is that we don't let them grind us down and spread all over our life, and the above is how we do it.