Having gone down this road with my child, unless it is a goalie specific clinic it is a waste of time and money. Goalie specific clinics are good. However the very best way to start him out is with private instruction. Maybe you can get the local varsity or JV goalie relatively cheap. In order to get the most mileage out of goalie clinics you want him to show up there with some foundation for them to work with. Specifically learn how to do a ball toss, stepping and how to "walk the line". All things you can do at home with him. Once he's got the basics of the technique down you can progress from there. Then he'll be ready to start getting shot on. The art of making a save is all muscle memory and repetition.
Hope that helps! Good luck to him!