BC will cover the material at approximately the speed of a college calculus course, while AB covers less material at a slower pace.
On the other hand, plenty of students go straight to multivariable calculus and do well after a 5 on the BC exam.
What I would suggest is that if (a) he has an AP calculus score that allows him to skip one or more math courses in college, and (b) he needs to take higher level courses that depend on those math courses, that he try the college’s old final exams for the courses that he is allowed to skip. That way, he can find out if he knows the material well by the college’s standards, or if he should review a few topics, or if he should retake the course.