PT when this kid throws to get loose or throws from another postion how does he grip the ball?
I am in favor of changing him unless he was flat out dominating with the three finger. We all have relative knowledge of what the two finger grip will do and that is why everyone teaches it.
When I picture him throwing a three finger I just automatically envision a fast changeup or a hittable changeup.
Something I heard one of my coaches one time tell one of our pitchers was if you want to change a grip for pitching then work on it first when you throw to get loose and long toss (as long as you keep it realistic effort and distance).
I bet he throws to get loose with three fingers (but may be wrong) and if he does then have him switch to two fingers here. He will accept the change easier here than he will if he hops on the mound with a new grip. It gives his body / mind a chance to make a change.