Neither.
Simply by being admitted to these very selective schools your son has shown he has high potential. Each of these schools are wonderful learning environments and have experienced premed advisors. But where the rubber meets the road is what your son does with his opportunity. Does he work hard for good grades, make a favorable impression on some profs for strong letters of rec, get exposure to patient-care?
As Amherst said in its premed guide
Amherst has no magic touch that automatically elevates a student’s chances of entering medical school simply by virtue of the student’s coming to Amherst; no college or university has that kind of magic touch.