I find it astonishing that people seem to think a 17 year old should only be focusing on one martial art if he plans to open a dojo. At 17, he's probably got multiple years before opening one, and the things that will make it successful have a lot more to do with stuff like what
@drop bear was suggesting...some business classes, teaching classes, and possibly sports medicine course added in for good measure.
As for martial art, he can change his mind about his primary martial art 3 times in the next few years, and still train it for ten years before he's 30. Or he can continue with all 3 if he wants. I'm pretty sure each dojo I've been too, even ones devoted to one style, the head instructor had trained in at least 3 styles at some point or another. And, again, he's 17. This is like telling someone not only should you have your major picked out, but you should know what subset of that major you should be focusing on, and ignore any electives or anything not related to that once you start college.