Hi! I took online math in high school via UCCS. Their professors and TAs are great! They have a TA/grader committed to just the only HS students and the professors always respond to emails within a few hours (even minutes at times). They’re also really great about rescheduling stuff around his HS and AP schedule.
Highly recommend the program. Helped me a lot and I loved the classes!
Link: http://www.uccs.edu/~mathonline/
Good luck to your son!