mtw, I didn't want to make you feel depressed! Certainly not, but I just spoke my mind. I just had a look from a different angle, that's all. Don't think that you are a bad mother! Bad mothers wouldn't care.
I understand that it is kind of difficult to get 'in' with the crowd if that crowd is so close-knit... Can you yourself do something and then lay contact with people and meet up maybe? I don't know, knitting club 
or whatever other mums do in that place... I mean even if they work all day, then still I hope they do stuff? That way, you get to know them, they get to know you and then you can maybe meet up 'casually' so your son doesn't suspect anything? That's only an idea though, I can't really smell your situation 
.
Anyway, what ragged said is probably a good idea, but I don't know if you wouldn't end up blening stuff forever... He should try.
I have heard that not eating veggies or fruit could make you feel depressed. No offense to you, but him not wanting to eat them is of course a vicious circle if his eating habits are connected with depression (which I don't say it is but I have an inkling only, I am not qualified in this). Believe it or not, my hubby and I went to Poland, didn't eat a vegetable for a week and felt so miserable, until we had some green runnerbeens. That was fab! We felt so much better! By which I want to say that your son might be stuck in a vicious circle. It needs to be borken and that is hard, but rewarding afterwards! For everyone!
Anyway, so, tomatoes are happy veggies. Does he eat tomatoes? I guess he doesn, because he eats pizza. You can do loads with tomatoes (they are a start): soup with meatballs (mmmmmm 
), salad (though that will not be a success, I guess), pasta sauce, just tomatoe sauce over other veggies etc.
Maybe, like you say, he is a small eater. Then there is no reason to force him. If he just eats and is not extremely skinny, it shouldn't be a worry, but I would definitely work on this.
I have seen in such programms on TV that they gave children (a lot younger) something they did like and something they didn't, say whole broccoli pieces, boiled. The thing they did like was allowed when they had licked the broccoli or whatever. (please, do ick two things that are compatible. Not banana and meatballs 
) Is that not something to try? If he smells it, great! Applaud. (If he doesn't like this, then don't. I would have hated it. Then just give him a reward he likes. No sweets though, but something he likes, I am sure you know your son better than I ). And a reward. Then if he licks it, great, another reward! Then he will take a little bite, great! Another reward. And by the end of a week he should be able to eat that particular thing.
Also, make things tasty. Broccoli steamed is much better than boiled to bits.
Actually, try sweet tomatoes first. Cherry tomatoes are great. Does he like fruit? Try that too. If he likes the taste (so drinks smoothies), then there is nothing but the texture that holds him back and that is not absolutely impossible to tackle. The only thing is try, try, try and try some more.
Try carrots. Do not boil them, that's juck. Try to cut them fine, in small pieces, sweat an onion and then the carrots, with a little sugar (coffee spoon at most), add some water (small amount), lid on the pan and wait until cooked. That is jummy. Sweet is always good with children.
Cauliflower with white sauce? If made well, that's also nice.
Sweet peas.
I don't know, you know him better. Also try some herbs. He is old enough now not to worry about it. Makes food more tasty.
Do you know whether he actually has any real friends, or does he only 'know' people? Otherwise, ask them to your place?
If he doesn't like out of school activity, then do not force him. I never saw the use of it and thankfully my father was also a loner. If I had asked, though, I would have got it, but other than that, I wasn't interested. That in itself is not really a concern.
Otherwie, the school might be able to help as they can actually observe his behaviour in school, whether he is bullied or whatever.
Do you also know that he doesn't eat too much at lunch so he can't actually eat anymore in the evening? I mean, that's well possible. So if you then, on doctor's adivce, hunger him out, it won't really help as he still gets his lunch the next day. And, if you don't mind me asking, do they not have anything apart from chicken nuggets or burgers? Then it might be a wise idea just to make some nice healthy soup when he gets home or so to get the happy vitamins in him ...
Good luck! Let me know how it goes.