He doesn't sacrifice his in-state ties by going to BYU. If you are from NC, he will still be considered a NC resident when he graduates from BYU. This will be important because UNC does prefer in-state residents for the medical school (if he wants to come back to NC for medical school). This is also the case at ECU. This is doesn't matter for the MD programs at (which takes students from all over) or Wake Forest (takes a several BYU students every year).

UNC has a great national reputation and will prepare him well for medical school. But so does BYU, and BYU has several intangibles that aren't available anywhere else. More important is what he does while he's in school.