My son is an aerospace engineer/math minor graduating in 2020; he wants to become a fixed wing pilot. Which branch offers his best chances?

Navy, Air force and Marines have fixed wing pilots. Each has a unique culture. He should read up on and speak with the officers at his schools ROTC program. Determine which culture fits his goals and desires. Good luck.

Not the Army!!!!!

He might have a better chance in either the Navy or Air Force. And he should get in contact with a recruiter now to inquire about officer training and get the ball rolling.

Not the Army. We have a fixed wing program but it's rolled into our rotary wing program, so you don't get an actual choice on which one you'll be.