Keep it pretty basic. Stick to asking about his baseball program. What it takes academically to get into his school; who else they already have, and what year they are, at your position; any questions about the direction of the program and things like that.
Coaches realize you're 16 or 17, and they're used to talking to teenagers. Remember that if you're a prospect they're interested in, they'll be interested in creating further interest in their program from you. Most of all, RELAX and be yourself.