My daughter (D23) is like your son – very advanced in math. She started in AP Calc BC lat year as a 9th grader and switched “down” to AP Calc AB one month in. She had an extremely demanding schedule and was losing sleep trying to keep up with everything. She found AP Calc AB easy as pie, aced it, and is now looking forward to the BC class (it just started yesterday) as she feels very solid and grounded in the previous material.
I think the above outcome – a kid who got straight-As in all her classes last year and also got 9 hours of sleep most nights, plus had deep involvement in extracurricular activities – was ideal for her. Had she continued with BC last year, she may have gotten an A, but it would have come at the cost of a ton of unhealthy stress, profound loss of sleep, and cutting back on her ECs. She prides herself on her math abilities and was upset at first about “quitting” BC, but she got over that pretty quickly when her life went from feeling horrible to enjoyable.
Plus, calculus is a topic that takes time to sink it for many (most?). I don’t see the point of rushing a kid through it if the kid needs more time to truly grasp all the material. I doubt adcoms will see the point either. So now my 10th grader is taking Calc BC and will then take math at community college for 11th and 12th. I feel she has a very firm grasp of everything Calc AB, and if she had forced herself through BC last year, she wouldn’t have retained much.
I get wanting to know if getting an A in Calc AB is better than getting a B in Calc BC for elite admission purposes. However, for calculus, having a deep understanding that sticks with the kid is so important. For this subject, I worry more about making sure my kid has a lasting and secure knowledge of it, which meant for her slowing down the pace to allow for better and deeper understanding.