Princeton no longer has grade deflation. They got rid of the policy to cap A’s to 35% of each class. The GPA has been steadily rising. At this day and time, the average GPA at both Williams and Princeton is comparable (around a 3.4-3.5).
If Princeton fits his personality better…then, it should be Princeton. Not every student needs or desires the level of intimacy Williams and other LACs provide. Princeton, despite being a large university, offers one of the most undergraduate focused experiences, so he’s not losing out much in terms of making relationships with people there.