@Ghostrainstorm
You may want to look at these related threads.
https://www.mumsnet.com/talk/higher_education/4928594-maths-a-level-weakest-link
https://www.mumsnet.com/talk/higher_education/4894303-a-level-choices-to-keep-options-open
My DS is in a similar (ish) position, but has no idea on the future.
Firstly, Maths, Physics and ANOther is probably the best combo for keeping options open, as numerous people have mentioned.
Re FM, as per the first link above, consideration is needed re the ability of DC, as it's obviously hard!
Has DC done a proper Y11 mock yet (mine hasn't), to give current ability.
You mention a Physics degree - is that what he is saying, or just something using Maths and Physics (or Business)?
Putting M, P and B in Uniguide gives (best match) Economics, Bus Man, Comp Sci and Accounting, reasonable for those, as Business A-level is included. It doesn't mean you can't do anything else. If you were expecting 20+ choices then it doesn't work that way.
Putting M, P and FM gives Maths, Mech Eng and Comp Sci.
But DC could do Mech Eng with M, P, and ANOther.
Short answer, M, P and Bus would be fine.
Consider carefully the choice of FM (ability-wise), or do it as a 4th and drop (or Business) as required.