I think he should contact the universities (or Oxbridge colleges) that he's interested in and ask. Many moons (decades even) ago I did the IB and contacted all the medical schools I was interested in to check on their view of my Higher and Subsiduary levels subjects. They were very pleased to advise me on what they prefered - shows inititiative etc - Also some Unis/departments love the IB (and some are ignorant) and you can pick this up before wasting a choice on the UCCA form!
I've sent him a link to the Oxford page for PPE. They say they don't have any real subject requirements, but that 90% of successful applicants have maths to standard/As level, which he's doing.
Philosophy and/or economics are not required, but they said having one of them may be advantageous.
I then looked at various economics degrees, where they say that economics is not required as they teach it all from scratch, and philosophy ones where they say it may be helpful. So on balance I think it would be less a waste of time to do philosophy, iyswim.
But I've suggested he looks at the courses to see which one he thinks will be most interesting and he can be most enthusiastic about. If he gets an interview for Oxford (or anywhere) they'll be able to tell if he really loves the subject he's applying for, and I think it'll be better to come off well in an interview than to have a tiny advantage in the way a subject is perceived, if any.
The IB subsiduary level Maths isn't really A level standard - it might be better to see if his Maths is good enough to switch to Higher level if they are hinting at wanting Maths.
Also IB students do a lot of philosophy on the Theory Of Knowledge course. I would have thought that was sufficient - he would have lots of things he could bring up at Oxbridge interviews based on that coursework alone.