So Niche has a Politics on Campus poll that is included in college profiles. The response rate is really low, but my two cents is it often tracks pretty well with other information about these colleges.
In Amherst’s case, it worked out to be 9% Very Conservative, 45% Moderate, 9% Liberal, 27% Very Liberal, 9% I don’t care. Understanding those exact percentages are not going to be precisely accurate, I think this is probably indicative of a couple truths.
First, there are not going to be a lot of committed conservatives at Amherst, and they are going to be outnumbered significantly by the very liberal people at Amherst. And among other things, that means the latter may be a lot more visible presence.
But second, still the majority of students will actually fall somewhere in the moderate, to only mildly liberal, to really not that into politics at all, range. But even though they are a majority, they may not be so visible as such, because mostly they are just busy doing not political things.
I think all this–which is not at all unique to Amherst in US higher education–helps explain some of the contradictory impressions you sometimes will see. For some people, the very liberal/activist population may seem like it is a huge presence, but that is really because they are intentionally trying to be more visible. But other people realize that in truth most of their fellow students are not so much that, and for them that is fine. But then some people really want a robust conservative community, and then that is not so fine again.
OK, so on the one hand, I think your son would find most students at Amherst are in fact similarly not themselves “extreme” in any notable way. However, your son might want to assess how comfortable he would be with a visible very liberal activist minority (whether that is “extreme” being in the eye of the beholder). If that still sounds OK, though, then I would not be worried about him fitting in comfortably at Amherst.