If you are hoping to use AP or college credits to shave off time from college (only noting that your son has taken very advanced math not often offered at high schools), schools like the Claremont schools and Amherst don’t generally accept those credits, or very few of them. Also, the obvious CS choice of the Claremont colleges is Mudd for a CS major. However, the CS lectures are pretty large for a LAC. Now I will say that my kid is in a sort of dual Physics and CS major there, and while the class size is large for her CS classes, she has not had any trouble getting more individualized help when she has needed it in CS classes (and she did not come in with much CS background). Also, at Mudd your kid would take a math placement test. He may or may not place out of the math he has already taken there – you have to be really top notch in the placement tests to do that given the background of most of the students there. I don’t think the CS class sizes are smaller on the other 5C campuses, either (my kid took one class at Pomona, and I think it was about the same).
I doubt you will find many colleges that are strong in CS with very small CS classes, if that makes sense. The smaller class sizes tend to be more in the humanities side.
I am not clear why you think small classes would be best. My kids both have attended LACs, so I see some of the benefits, but your reasons aren’t too clear to me. Is it because your son tends to “hide out” or fly under the radar in a larger group, and won’t participate? Is it because he LOVES to participate, and thus the smaller group is very much to his liking because he can do so? Is it because you want him to develop closer relationships with his profs? Is it because he has some kind of academic struggle that you worry will go unnoticed and cause him to possibly not do well in college? Just trying to get at your reasons, because that might help us zero in on colleges that are a better fit. Really, most STEM courses are not taught in a seminar style anyway.