I followed a similar path to Btown to IU. I attended College Wood elementary from k to 5 then went to Our Lady of Mount Carmel for 6-8. I then attended Brebeuf for high school. If Cathedral was his choice for high school I suggest you take the college decision out of his hands. I kid.
Anyways I had no problem adjusting to IU freshman year. The size thing is overrated mainly because you are only around your dorm floor pretty much anyway. Also Brebeuf, Cathedral and schools of their ilk all do a good job of preparing you for the college experience.
I also know a couple people in the music and education schools. My RA this year was a sophomore in the music school and its top notch and he really enjoys it. Take the Geez Jr. and the rest of the family down for a football game this fall. The campus is beautiful that time of year and plus we actually might have a pretty decent football atmosphere next year. However if you can score tickets to a Bball game next year then you might be able to convince him to go to IU on the spot if he enjoys sports at all. I see several advantages for you and him if he attends IU. Good music school, and I could see the costs being very little. He is an in state student to begin with and from what you have said about his grades he has similar qualifications to some of my friends who pretty much go to IU for free. He would be accepted into the Honors college immediately and could be a fantastic candidate for the Wells scholarship. Plus if he gets to IU and doesn't like his original major choice he can change to another one easily. Like I said earlier there are probably 20 people in my graduating class that are transferring from their original college choice to IU along with the nearly 40 or 50 that attended IU starting freshman year. Mind you my graduating class had like 192 people in it.