Did he already take biology? If not…and he plans to take biology, he should take the most rigorous offered at your school…in my opinion.
Honestly…I’ve never heard of “AP prep” before.
He will also need a top SAT or ACT score.
There is a question on the counselor recommendation regarding academic rigor. You want that school counselor to say he took top rigor in terms of courses. That does NOT necessarily mean he needs to take all AP courses.