I interviewed at Radcliffe but was waitlisted. You must be so proud. Harvard opens so many doors around the globe. I've seen the camaraderie. Harvard grads strike me as very real world oriented compared to Yale and Princeton. It may not be accurate but it is my impression.
I interviewed applicants for law school. The best advise is to just be yourself. He should take the attitude that Harvard would be lucky to have him as a student.
Of course, if he is selected for a Harvard interview, his world is full of the widest options. Even if he were blessed with a very high IQ, he had to dedicate and discipline himself.
Just the interview is a major life achievement! If it were my grandson, iI'd hire a plane that flys at the shore with banners trailing.
I am so happy for your family. My best wishes are with him.
P.S. Isn't it nice to have a proud gm than one who feels education is a mockery and evil.