I wouldn’t worry about schools just because they only have 4-5 professors in CS. It’s likely that your son will be in relatively small classes with a lot of personalized instruction, which is a good thing. The main issue with the smaller schools is whether they offer the classes your son is interested in. I’d look at various schools and compare their CS offerings. Some schools may offer tracks or specialties like cybersecurity, AI, games, etc., that appeal to your son.
Not an LAC, but if your son is applying to places like Michigan, CMU and MIT, might as well look at Case Western. Very few LACs are well known for CS, mostly because CS programs build their reputations on graduate-level research and on how many successful tech businesses were created by alumni.
And I have to chuckle at the idea of avoiding professors who got their degrees back in the 1990s. The '90s weren’t that long ago. I personally would be more cautious about professors who got their Ph.D.s within the last five or six years, due to their general inexperience.