Computer science would be better than Physics as quite simply on a games design course they will be using computers a lot - and coding using game engines. Even when not writing their own “scripts” in a programming language they will be using quite technical computer applications.
Physics will be useful only as a foundation for the Computer Science of games that use physics engines (not all games do).
Games are of course software, and the best digital artists in the software realm have a good basic understanding of programming (computer science) and maths. For example creating 3D graphics can require quite technical skills and understanding of vectors etc.
I suggest he looks at the prospectus of universities that offer a degree in Games Design to see what the course content is and then choose A-levels that will support.
It’s most likely that he’d be using the Unity game engine, but Unreal is another possibility. If he has a decent computer he could start getting to grips with one of these before the degree course and that would help him hone in on his preferred area. If he is more into the art side then he’ll likely spend his own time doing art… teaching himself ZBrush, PHotoshop etc.
If he decides whilst doing his A-levels that he prefers coding to art then his Computer Science won’t be wasted and he might pivot to Games Programming.
If he decided whilst he is doing his A-Levels that it’s definitely more the art side he loves then Photography would not be wasted. Even doing 3D gfx requires a good understanding of photographic principles.
So I would say Maths, Computer Science and a more creative subject such as Photography would be more balanced.
I worked in the software industry for a few decades.
I think my son might be in a similar position in a years time and the above will be my advice to him (I’ve already given him a taste of Unity and ZBrush at home). I advised my son against GCSE Computer Science as it was too vague and basic to be of any practical use but A-level CS will be necessary if aiming for degree courses that include coding/scripting which both game design and games programming courses will.