There are quite a few things about the Cambridge system that aren't that great, I think, and one of them is the short intensive terms. They don't suit everyone and if he was exhausted from his A levels, he might feel that it all seems too much this year. The first year marks don't count though, if I'm right, so my guess is that often they are marked quite harshly because they don't want to be giving away firsts early on, but if you look at the stats, then Cambridge does indeed have a very big proportion of first class degrees, around 30% or something like that so there's no reason to think your son won't do well later on.
He sounds tired and fed up, I guess he has exams in the next week or two. I'd be sympathetic, try to keep him going and keep his spirits up, and review the situation once he has finished the exams, enjoyed the rest of the term and then is back for the holidays.
Oxbridge isn't for everyone, it's quite old-fashioned IMO in the way they tutor and pressure the students to work intensively, it works well for some but not for others and I teach students who have dropped out of Oxbridge and taken some time out then come back into a different university, just make it clear to him there are options. You might find this is just a moan, you might find he wants to leave, see how it goes.