Thank you for all the helpful posts. It is so useful to have that longer time horizon perspective from others. I'm making a mental list of stuff to do - write letters and post them , re-visit the tea-time period, and get something else into my life!
I don't have any reason to think he is less than completely happy. He says he is happy, he writes letters enthusing about the "lots of fun" that he is having, he is doing well academically (and I assume he couldn't be doing well if he were unhappy).
It is just that, in the beginning I had the impression that it was tonnes of fun and that he found it a bit like a holiday camp with a few lessons thrown in (just so they don't get overtired from all that sport!). Whereas five months in, things are different. Last weekend he had a cold and couldn't play the sport (which was mostly cancelled due to weather), and I just felt upset not to be available to him when he was slightly unwell.
stoppingat3, I felt just like that when DS1 went off - the house became much more peaceful as DS1's behaviour was sometimes quite challenging. But DS2 is quite and lively in a very desirable combination, so I am finding his absence harder.
Also, I used to pick DS2 up from his extra curricular activity and bring him home, DH usually cooked. Now that he is not here, I don't even know what time to leave work. 
Thanks HG for all those insights. I know very little about real boarding schools or what goes on there. (I am discounting information obtained from those Malory Towers stories several years ago!)