Stands by for potential flaming ...
Biology is much easier than Chemistry, given your son is doing Maths and Physics it would suggest he is a smart lad and strong Maths will help a lot in Chemistry. My daughter did Chemistry (for potential dentistry school) and she struggled with it as its tough and her maths wasn't really strong enough.
So if your son is uncertain and worried about grades take the Biology and take some stress off. Yo might want to think what he will do as his 4th subject in first year, how about something like Economics - gives him some more flex for second year choices (he could drop say Biology/Chemistry and keep Economics) and might be more useful in terms of career choices.
You want to make sure he's got subjects he likes, he is good at so he gets good grades. Have a look at potential degrees and what the subject requirements, look at likely good Unis he will aim for as the better ones will likely have quite specific subject requirements
Best of luck to him, I don't envy the kids today after all the pressure of GCSE you then have AS1 and AS2 and Uni applications, its a tough few years and at a transitionary phase in their life.
EDIT I did maths, further maths, physics and computer science - I've worked in banking/finance for 35 years. Quant skills are always in demand