Oh, this is so tough. My heart really does go out to you for how this must be making you feel. I know that at my DS's school they are big into the whole Restorative Justice thing that a lot of schools do. Basically, the way it works is that two parties are brought together to try and work through a situation and find our what went wrong and bring some resolution to it. Often it happens if a particular incident has occured, such as a fight or an argument, and on many occasions there are more than two people invited to attend. The idea is that the situation is examined from both sides and the parties are able to put their viewpoints across and say how it's made them feel. I think the idea is that the whole thing gets "thrashed out" so to speak and each side gets a hearing and hopefully differences are resolved and matters laid to rest so that people can move on. They have staff specially trained to deal with these sessions. I have also known it to happen in sitautions where there have been fallings out with groups of friends or "ganging up" and it's left one party feeling uncomfortable, confused or upset - as is perhaps the case wtih your DS. I was wondering if his school might be able to get your DS and the boys he's no longer close to together in a room to discuss where things have broken down? At our school they are keen to get all parties to understand and acknowledge how the other is feeling and the impact that their actions have had on the others. At least if these boys explained to your DS why they no longer want to have him as a friend he might be able to make a bit more sense of it all..? It might also be a case of one ringleader and other "sheepole" as I like to call them, following. Maybe they have no idea how their actions are making your DS feel and being forced to confront it might just make them think...? Maybe your DS did do something to offend inadvertantly and and didn't realise? Who knows? Maybe he did nothing at all but at least thrashing it out might give him some answers. It's my feeling that ignoring someone who you have simply dropped as a friend, and doing so as a group, is a form of bullying and the school should want to investigate this, particularly if it is making your DS miserable as this will no doubt have an impact on his learning.
The years between 12 and 18 are such odd years. Children are all growing and developing and changing, finding themselves etc and fall in an out of friendships regularly. Sometimes, particulalry with boys, some are interested in the opposite sex at a far earlier age and others are slower to develop in that area - this can mean that all of a sudden they have little in common with boys they've been friendly with for years. There may be all sorts of reasons why his friends have dropped your poor DS but it would be lovely to think that he could find some new ones to replace them. It's very hard for you as a mum to watch all of this and to see him go through heartache but all I can suggest is keep the lines of communication open with him and keep telling him that he can always tell you anything and that you're there for him if he wants to talk at any time , day or night and that if he feels there's anything you can do to help as far as the school goes, just tell you. My feeling would be to back off a little from trying to organise him and his social life for him. The last thing you want to do is to irritate and send him into his shell. I think if I were in your shoes I'd contact his form tutor and try and have a chat to see if they can help in any way, or at least give you any insight they might have into how he is at school.
The bottom line is that a lot of kids look back as adults and say they hated school because it's a pretty shitty time for some people and growing up is hard and fraught with problems but you'll see, he'll find his own way and won't be short of friends for too long. He will find someone, or a new group, to click with sooner or later. It's just a case of finding them.