I also do not think that a 4 on a humanities related AP test is going to have any significant impact on your son’s applications. MIT by the way does not admit by major, so your son’s choice of major similarly will not have any impact on his chances at least for MIT. My guess is that Caltech is probably the same in this regard.
However, as @eyemgh has correctly pointed out these schools are reaches for essentially all applicants, and the very large majority of applicants are very well qualified to attend and do well. Your son definitely also needs to apply to other schools where admissions is far more likely.