Ask HN: Self-assessment and self-preparation for teen who will study calculus?

I'm a mathematics teacher by occupation recently, but the pupils I teach are of elementary school age, studying topics preparatory for studying algebra (in other words, the mixed bag of topics called "prealgebra"). An alumnus of my program has just taken his high school's tests for credit by examination for the high school's course in "precalculus," and passed those. So he can surely take the high school's AP calculus AB course next school year as a tenth grader. He is also interested in taking the qualifying test for the University of Minnesota Talented Youth Mathematics Program (UMTYMP) and will take that test soon. I'm fairly aware of how he has studied to get ready for calculus so far, but I thought I should ask among knowledgeable people what else you would recommend for a teenager who is self-motivated, curious, but possibly still in need of thorough review to make sure he is a successful student of calculus next school year. If you have any recommendations of online readiness tests, self-study materials, books (I already have a LOT of those), or other helpful resources, I'd be grateful if you told us about those in replies to this question. I'm especially curious about resources that deepen the mathematical understanding of a young person passionately interested about biology, which is this student's background.