I should think so, but not in isolation. I do not quite understand the timeline, is he just finishing up his junior year?
First off, he should aim for 5s in ALL his STEM AP tests, not just the minimum of 3, because he has to submit all of them, no choices.
Also, if he takes more STEM AP classes senior year, he will have to list them in his application and Oxford or Cambridge will want to see 5s in all of them, too, predicted by his teacher and confirmed by the actual exam. No dropping classes or choosing not to take an exam or submit results. Again, no choices.
On the one hand, that is a risk, because that way, his offer will be conditional on getting those 5s.
On the other, he will have to take a university administered aptitude test in November (I believe that for engineering, you take the physics aptitude test) and pass an interview (which is much more like an oral exam, having to convince actual engineering profs they want to teach you) in January. (This is for Oxford, I am not as familiar with the Cambridge timeline, but believe it is similar). The more physics he’s got under his belt for that, the better. That way, it’s another semester.
Personally, I’d recommend going all out, as much physics as he can, and trying to excel in all of it. No hedging bets. Oxford and Cambridge want to see the candidates who can’t stop themselves from taking physics!