If your son would feel more comfortable retaking calculus in college and his schedule/finances allow him to, then let him take calculus again (provided his school allows him to take it for credit–not all will if he gets AP credit for it). At worst, it will be a review of material he knows but will give him time to adjust to college courses (in terms of grading, tests, pace, etc) in a subject he is familiar with. If he finds it easy, it’ll be a GPA booster and will allow him more time to focus on his other courses, get a job, join a club, or just generally get acclimated to college in his freshman year. Retaking the course and finding it easy may also give him confidence in his academic and calculus knowledge and ability. And if he finds it challenging, then it’ll give him a chance to really learn the material before moving on.
How well he does on tests may not always be indicative of how well he knows the material (speaking from experience–I took physics in high school and college, 5’s on the AP Physics C tests, got A’s in every class, including college physics, and I didn’t understand any of it then and I don’t understand any of it now. I think I was just a really good test-taker with a good memory), and his desire to retake the course may reflect that. Or it may reflect that he doesn’t have as much confidence in his ability in calculus. Or he may just be saying this to take the pressure off for his AP test–it doesn’t matter if he passes or not if he’s planning on retaking the course anyway. No matter what the reason, unless you have a really good reason to discourage it (financial concerns, for example), I’d recommend supporting whatever decision he makes.