I second Schutzee's recommendations. You may also wish to consider what airline you prefer whenever you ever have occasion to fly, since presumably the card will be in your (parental) name, not your son's. However, an airline offering the most convenient connections/non-stop flights to wherever your son is going to school is probably the primary variable, especially since most travel will be at holiday time.

You should also try to use the card for as many other purchases as possible, beyond just tuition, in order to get extra miles.