Some very strong CS programs fall within the engineering schools of major universities. Not all, of course, but a good number of highly ranked CS programs are. So if those happen to be the programs you’re targeting (or if you’re still trying to keep the most possibilities open), your student needs to be keeping up with the engineering hopefuls as best as he can. Basically they’re the competition for those particular seats. That means taking calculus in high school as well as having a high level of rigor in other classes too.
Even if those aren’t the types of programs he is targeting, he will still need higher math in college, probably including discrete mathematics for CS. So you would be doing him a favor to strengthen his foundation now. Depending on your budget that could mean a tutor or Khan Academy and more office hours with his teacher.