If I were you Kez, I would be telling him to to agree to anything at the meeting, but to say he wants to come home and discuss it with you, after the interview and that he will then ring them later in the day.
Physics at A Level is obviously very reliant on maths,so having a C in Maths and not meeting the requirement in science either, means the college are right really to tell him it is not a good idea. He could ask though, what someone with his grades would be likely to get if taking physics A Level.
Whatever other options he is offered, he should ask, what he would be predicted to achieve given his GCSEs and what people with those grades go onto do and where, after the Sixth Form. I would advise that he writes it all down, so he can report back accurately to you, rather than just signing up on the spot.
I say all this, because big decisions are about to made and I don't think most 16 year olds should be making them alone at an interview, but need parental input. You can be looking into university courses and whether the courses he is being offered will be acceptable.
If when he returns, you don't feel you have enough info, ring the course tutor for further info.....don't feel that you are unable to do that. better to push for adequate info than to make a mistake.