Hello
Two things to consider really.
First: is physics relevant to what he wants to study at degree level? Obvs he might not really know yet. But eg if he wants to do a hard maths/STEM/medicine subject, evidence that he aced physics would be useful.
Second: for Oxford specifically, what matters is how you do at GCSEs compared to the average GCSE results in your school in your year. If the average attainment at your school is 4s & 5s and you got 7s and 8s, you're laughing. If the average attainment at your school is 7s and 8s and you got all 7s, you won't score so well. It won't knock you out of the running, but you'll have some catching up to do on other aspects.
So far Oxford have regarded 8s and 9s as the same when scoring GCSEs. (No guarantees this will be the case for ever of course.)
Finally, the best way to guarantee you don't get in is to not try! If he goes for a subject that has an entrance test (which lots of them do), knocking it out of the park on that goes a long way to getting an interview even if your GCSEs weren't great.
My older DS had 5x7s at GCSE (he got four 8s and 9s, including maths and physics) and got in for maths. Younger son got a 6 in engineering and is now off to Oxford to study... engineering! (Albeit with 8s and 9s for the rest of his GCSEs.)
Don't let anyone tell you you have to have all 8s and 9s. You absolutely don't.