So having a just sent a 18-year-old off to college myself, I’d say that we found a happy medium somewhere in the middle for him. I feel the kids need to have a life outside of school, i.e. they need a child-hood!
My kid went to some structured activities, and he had some time to burn on his own. He was forever doing his own projects beyond school/structured activities - which has led him to wanting to be a film maker.
Like yourself, he had some tendencies towards being an introvert. One of his activities was scouting, where he learned some leadership skills. Having a chance to practice them helped him to be much more confident in front of people, i.e. helping him get over his fears of public speaking, etc. He eventually became the SPL and got even more experience herding cats. ;-)
He is as ready for the outside world as we can make him. Time will tell how successful we were.
Looking back on my own life - I did just enough homework to get a B average in High School. Usually I did it between classes. When I got to college it was a real challenge to learn study habits. That was one of the things I wanted my kid to avoid. Like I said - happy mediums are possible.
Happy mediums are absolutely achievable and necessary for teens. Introverted teens often become listless and get into dead-end hobbies like video gaming. I am a video gamer now, but when I was in high school, video gaming consisted of my Nintendo and the neighbor kids. Nowadays games are so involved that kids become lost in them (think: World of Warcraft).
I was also a Scout and found scouting to be a necessity as a young man. My yet-to-be conceived children will be scouts/brownies as well. It breeds leadership, personal responsibility, and social skills that are becoming a rarity among kids these days.
I bid your son the best of luck. Today’s colleges are intellectual minefields. I’m 10 years out from graduation as an undergrad, and the academe tapestry is no longer one of universality but progressive specificity.