Frankly, if your son is happy where he is, then don’t push it.
Looking at Exeter just because of Zuming Feng… Well, I remember talking to one of the late Vaughn Jones’ grad students back in the 90’s. It’s a big feather in one’s cap to be able to say that one has a Field’s Medalist as one’s doctoral advisor, I guess, but it sounded like he was advisor-in-name-only, because he had little time for his advisees (over committed). It did not sound like the mentoring relationship one hopes for.
It sounds like you have a great educational set up where you are! Your child has excellent resources AND personal attention. It does not sound like he’s a big fish in a little pond - more like he has access to a lake!