Thank you for all of your responses. I will be back to read and digest. Quick answers to some of your questions:
-He is extremely introverted, loves the outdoors and biking, is musical (currently taking guitar) as well as artistic, and currently loves adventure novels. He often, ever since he was small, chooses one subject and delves into it until he knows everything about it--when young, dinosaurs; later, Harry Potter and other full series of books, WWII. His personality--which we LOVE--can be off-putting to some, especially teachers, because he does not often share what he is thinking/feeling.
-Our local elementary school splits into three zones/schools for seventh grade and he will be in a different zone than many of his very close friends, one of whom he has been with since nursery school. We are in a relatively small city, though, and we will continue these relationships through church and sports as well as informally. When I think from his perspective, though, I know that this feels very scary.
--His older brother, our oldest, is in the ibmyp program and it also seems to be a logical choice based on this second child's interests. It is a more international group of kids than the "regular" school would be, but it has seemed to be a positive environment for our oldest son--great peer support and good attitudes, as well as some wonderful teachers. Our second son does not, however, have any close friends from his current school who are choosing this option.