When you talk about "school" are you talking high school or college?
Not that it matters. NROTC is not a prerequisite for becoming a SEAL.
He is Army ROTC at a university where there are only two choices, either Army or Air Force.
Maybe he's going to the wrong school. If he wanted to be a doctor, would he attend a school with out a pre-med program?
I believe it is possible to switch over from other forces to become or try out for Navy SEALS. I'd suggest he talk to a recruiter. Maybe even a Navy recruiter.
Keep in mind..if he goes for SEAL's and fails out of the program..he will get the Needs of the Navy job...which could be anything. They won't just let him out of his contract if he fails out of the program.