I don't advocate anything - that is up to them and their parents. I've learned a long time ago to do my job and keep my nose out of anyone's business.
However, the more sports a young kid can play, the better they will be. Not only will different skills be learned but they will:
Learn how to win.
Learn how to lose.
Learn how to compete.
Learn how to deal with "pressure".
Learn how to deal with different coaches/styles.
Socially they also grow - with the way things are now, kids just don't play anymore. They don't know the pecking order and how to interact with different kids. In sports, kids are more exposed to other kids who have "different" families or social status; they now are working together and competing. I've gone to parties in college with other players I NEVER would have talked to/hung out with - and they would have never hung out with me.
So I think the kid wins on two levels: physical development AND social/emotional development. However, like I said before, I am my own children's "keeper". If I get asked, I give them my opinion but other than that, I stay out of it.
Your son is a junior now and has probably been doing sports awhile - hell, for 2 years of my life I played 5 sports every year: football and Cross country and track/baseball at the same time. By the time I was a senior I played football and ran track/threw discus. HOWEVER, I was insanely dedicated to training and never, ever took a day off.
So while you may want him to run track - as long as he is 100000% dedicated to the real goal, I don't know if there is a problem at this point of his life. But I think a lot of kids don't quite understand that once the opportunity to play X or Y is taken away from you, you'll NEVER get it again. Literally, never again.