DS1 did History and English A levels last Summer. He got A*s in his GCSEs in both subjects (100% in History), they were his favourite subjects at school and he has a great interest in them. There is a lot of work to do in both subjects, it was essays and reading constantly, there was not a lot of time for going out and they needed a lot of dedication.
DS2 has just started Biology A level, he did separate science for GCSEs, he got 3As. He is doing Biology, Chemistry, Sociology and Psychology. He loves Biology, he wants to go into Nursing, but of the four it seems to be the one where he gets the most homework and the most reading.
I think as long as your son is interested, motivated and prepared to work hard he should be fine. I agree with careers a couple of DS1's friends did really well in their GCSEs, then didn't work at all in Yr12, so had to re-do it because they did badly in their AS levels. As long as he is consistent he should be fine.
The other thing is, when my 2 chose their A levels they gave the school an idea, but they didn't have to confirm them until results day so there is usually scope to change your mind.