True for Utah, not true for Wyoming (only way to change residency is for parents to move there or be an independent student). However, Wyoming is fairly inexpensive for OOS, and there are OOS scholarships (Rocky Mtn Scholars program) automatically awarded for gpa/scores.
There are small D1 schools. Presbyterian has 1200 students Even the service academies only have ~5000 (although those are ~5000 pretty athletic souls). D1 does not mean big student population, but I agree that if he’s open to a ‘regular’ major (bio, chem, history) and not something super specialized, he’ll be fine.
It sounds like you live in the mid-Atlantic area? Does he want to stay around there? W. Virginia, Marshall, Towson, James Madison?