Scatter, I sympathise. Also have Yr 7 son. He came from a small close knit community primary school, and was sad for a day at the end of term. His new school is also quite small with similar ethos. His close friend emigrated at the beginning of Yr 5, and they Skype a lot and we have visits here and there. So no close friends going to his school.
However: I have friend with an older son who has just started 6th Form college. He is apparently quite gifted in his subject (Art) and is doing level 3, (none of this means much to me, am still in Yr 7 mental mode). He is a lovely sensitive 16 year old (a lot like a bigger version of my son). He started college and texted his mum within hours that it was disappointing, that he was the youngest in his class and that everyone else was far older and didn't talk to him. Same thing for next few days, he very unhappy, his mum distraught. The kids his age were in a different class doing level 1 Art, which he had skipped due to being specifically talented, incipient genius, etc.
I saw his mum yesterday. Her son is now fine. He has made friends with the older people in his class, but was surprised they would talk to him given the age difference. He has also made friends with the kids in other classes. The work is fine and he is happy. His mum told me she was very worried and upset, and that he said to her he felt under pressure because she was so upset.
Re your immediate situation, it depends on your son's personality and your relationship with him and what kind of mum you are. Each to their own.
Going to secondary is a huge transition, and I think we take it for granted that they will just adjust. It sounds like he misses his old school but I bet if he went back there he's see it as babyish and he's feel very mature and too old for it now. Now even though he chose it and it's probably the perfect school for him (because you both thought so at the time), he feels a bit overwhelmed. Or it's not what he thought it would be. Or whatever. As long as he's not being picked on/bullied and the school is capable of teaching him, and he is in a safe environment, then you have done what you can for the moment. He has to do this, not you. At this stage they have to 'do it' for themselves. Isn't it a good thing that he can't tell you anything specific -implying there isn't anything specific which is an issue, so no horrible kids, no nasty teacher, no girl who has broken his heart already? Just the unfamiliar, maybe...
Re work, are they not all just starting them off gently before they start giving serious work? I've noticed the standard of work at my son's school is not exacting but he does say he is learning things. Will there be streaming?
So, what can you do? Keep listening and try to get him to keep talking. Communication is so important. Try to identify exactly what his issue is. Think realistically about what you can do about it, what he can do about it. Lots of kids have teething problems at first. He is not the first nor will be the last.
He is still the bright, confident, happy, able boy he was 6 months ago but he has just faced probably the biggest challenge of his life so far. It may be, reading back over your OP, that he misses not having a friend. And that is something he will have to find for himself.
Me? I'd try not to give the 'you need a good kick up the backside/there are children in Africa who would give a leg to be in your school' speech just yet, although I'd feel tempted eventually. Remember: you chose this school together.
Your DS is having a lot of 'new' at the moment, so make sure he has plenty of 'old' as well to counteract that. I note my son takes one huge developmental leap forward and then two retrograde steps. I've noticed that since he has started secondary school he is relistening to Harry Potter audio books every evening (i could strangle that f***g Stephen Fry) which for him is a retrograde step, the old and familiar.
Hope you get some more positive feedback tonight. It's a fine line you have to walk, not making it seem like a big deal but also caring terribly.