Based on North Carolina General Statute 14-27.7A, the 20 year old man would be guilty of a Class C Felony. If the "victim" is 15 years old, and he is more than 4 years older than the "victim", but less than 6 years older, it is a class C felony. At the age of 16, the young lady can consent to sexual activity with this man. However, if what you say is true, it would seem that a Class C felony is being committed. It is worth noting that any conviction will depend on the evidence present. Just because this man is committing the crime, it does not necessarily mean he will be convicted.
Depending on the criminal record of the defendant he could face anywhere from 44 months to 210 months in custody, and he will most likely be forced to register as a sex offender.
This "gentleman" should stop any sexual activity immediately, and wait until this young lady reaches the age of 18 to avoid serious legal consequences. By the letter of the law, he can resume sexual activity with the young lady's consent at age 16, but the best way to avoid ruining one's life is to wait until the age of majority.
§ 14-27.7A. Statutory rape or sexual offense of person who is 13, 14, or 15 years old.
(a) A defendant is guilty of a Class B1 felony if the defendant engages in vaginal intercourse or a sexual act with another person who is 13, 14, or 15 years old and the defendant is at least six years older than the person.
(b) A defendant is guilty of a Class C felony if the defendant engages in vaginal intercourse or a sexual act with another person who is 13, 14, or 15 years old and the defendant is more than four but less than six years older than the person.
Mr. Howell has given an excellent and complete answer. It is indeed a felony, and upon conviction a Class C felony carries a mandatory active prison sentence. When I got cases like this as a prosecutor, if the age difference wasn't that great I would usually offer to plead the case down to an Indecent Liberties charge, which is a Class F felony. The ones who refused I tried to a jury and convicted of the C and they did indeed receive extremely lengthy active prison sentences.
Best bet would be to hold off until 16-- why take a chance like that which could ruin the remainder of your entire life.