Oh that's sad. You must feel helpless and sad for him. So many kids are miserable at school - even many of the ones who seem 'popular' IMO. :(
If he doesn't want you to involve his tutor, don't. His tutor couldn't do anything about it other than pity him, anyway - and he probably doesn't want (more) pity. It's not as if teachers can influence friendship groups - certainly not at this age.
Help him understand that school isn't the be-all and end-all of life. If it becomes a real problem for him, then you can perhaps explore major changes like transferring schools or home education, but at 15, he only has another year or so to go... Then he can move to a sixth form college naturally, where he'll almost certainly be happier.
Meanwhile, if he has good friendships outside school, I'd encourage these. Is there any reason why his 'weekend friends' couldn't come for tea in the week occasionally too?
Also, if I were you, I'd let him have an occasional 'duvet day'. For some people, an indulgent look-after-yourself day is all they need to deal with even quite difficult situations the rest of the time. I know some highly professional people - managers, teachers and self-employed business people - who take the occasional 'duvet day', and would argue that this 'battery recharging' is how/why they can out-perform their peers :)
Personally, I also always let my DCs have a day off school on their birthday if they want one, and I always book a day off work for my own!