If the son struggled with those types of courses in high school (humanities, social sciences) it is likely that he either opted not to take AP offerings in those areas and/or opted to skip the AP exams for those subjects and/or took the exams but did not score well enough to qualify for AP credit. I’m guessing it is the first - that he opted to take AP’s in STEM courses but not for English & History.
Nothing wrong with that - it is a legitimate choice in high school. But it does leave the student unable to test out of the requirements in college.