I'd be skeptical that your kid needs a curveball. I'd want to know a bunch of other information:
(1) How fast is your kid's fastball?
(2) Does he have good mechanics and conditioning to allow him to safely reach his maximum potential velocity? Or is he not realizing his potential?
(3) Is his change-up sufficiently slower than his fastball?
(4) How is his control with his fastball and change-up?
(5) How effective has the pitch sequencing been? If the coach lets the kids call their own pitches, that's a good thing. But have they been taught to effectively assess the batter's tendencies? Have they been taught how to properly assess the situation?
I feel all of these things are more important than adding a curveball.