If he wants to do anything physics-y at uni (or engineering) then he does also need maths. So maths and physics are a good pair to take together - and in fact, (perhaps depending on the exam board) there are some overlapping parts of the syllabus in physics and maths at A level so it can help.
But on the other hand, physics is really really hard! When my ds sat his A levels (inc Maths & Physics) in 2023, the grade boundary for an A in Physics was approx 50 per cent… the questions in the exams were so bizarre and difficult, and not straightforward at all. I don’t understand why an exam board would design an exam that the vast majority of students wouldn’t even be able to score 50 per cent on… it seems crazy to me. It’s a good idea to look historically (over last 4 years?) at his school to see what proportion of those taking physics get what grades. To do well at physics takes a lot of past papers and practice!
If he wants to do maths at uni, then I think a lot (??? I don’t know though, so you will need to check) will require further maths as well??? (But you need to check that).
Perhaps he should do a free online personality test (like a myers brigs), to see what that throws up as possible suitable choices?
Also as a pp mentioned, there is definitely a website where you can put in what you are considering studying and for what you can do at uni with it - is it something called university choices or something? I think there may be links to it on the ucas website??
A key thing is to keep as many of his possible options open (for uni study) as you can - while it’s not impossible (eg if you chose physics A level and not maths a level, then decided you wanted to do a physics degree, it may involve a foundation year first to make up for no maths A level)), it certainly makes life easier if you know what future options are possible with what combinations, and what future options aren’t.