I would ask what next years roster looks like. How many PG's do they take each year? How many players is he looking for?
If your son is going to be on JV, ask if he ever moves a player from JV to Varsity? If the coach initiated the call then it is an easier call, if you initiated the call then tread lightly, it has been a very trying year for all Prep schools and there is a chance next season will be the same.
Ask him about the politics within the school and how full paid tuition students are treated compared to students getting "help". If your boy is good enough to play at a Prep, but is not going to be the stud on the team, there will be ice time issues and who is paying in full and who is not. Gotta keep the full tuition parents happy so the money keeps coming in.