I work in aerospace related field and the number of engineers who also have a pilots license surprised me. I've had so many co-workers over the years who are pilots, I guess just goes along with aerospace and makes sense as a popular hobby for aerospace and mechanical engineers as well. Flying planes is a very popular hobby for aerospace and mechanical engineers. So I would take his desire to be a pilot as a sign aerospace or mechanical could be a good major for him.
It won't hurt ambitions to be a military pilot.
I actually had a friend at A&M in grad school getting his masters in mechanical engineering who was in the marine corps but super smart and went to fighter pilot training after A&M.
Also, aerospace and mechanical are nearly the same degree except aerospace has more focus on design of things that fly, like propulsion and airfoils/planes/lift. Mechanical is a little more general and fundamental vibrations, thermodynamics, heat transfer, materials, fluids…
Once in the workforce, mechanical engineers do just about everything aerospace engineers do, maybe with the exception in my experience being that most airfoil design is aerospace engineers. I'd say propulsion groups I've seen in general are like 50/50.
Job posts will nearly always say a degree in Aerospace or Mechanical engineering. Just the curriculum and specialization of aerospace major makes them more well suited for some roles, however a Mech-e who had some experience in that through clubs, hobbies, or electives would be qualified too.