Hi! This was actually the strategy used for UChicago. I am qualified and within their higher range/average, so the reach came from the very low acceptance rate. My school has amazing connections, and generally with UChiacgo, they accept a majority of their applicants in ED (which is why I applied ED and not EA). In the early process (I listed my schools above) I got accepted into a safety (UMass Amherst) and a far reach (University of Michigan). Since I already am accepted in one of the top in the country, I don’t see a harm in applying to T20s in RD. I applied to about 36-37 RD including safeties and T20s. This year has not been predictable so it’s very hard to judge based on past year stats.
Your son sounds very qualified, so even with a rejection from UNC he should still go somewhere amazing!