Actually, if he’s throwing what appears to be a curveball, he probably has a natural one, and I wouldn’t be too concerned about it. It may have a natural break, and I can just imagine that the batters who have been sitting on a fast ball are fooled by what he’s throwing. What he needs to do with this pitch is learn to change speeds on it. And he might even experiment with a knuckle-curve, which is a truly devastating pitch.
As far as the fastball is concerned—which one is he throwing, a four-seamer or a two-seamer? The latter is also known as a sinker, and a lot of pitchers who have trouble with the four-seamer will do well with the other pitch. Again, don’t worry about it.
One thing he needs to do—and this will enable him to get more velocity on his pitches—is something I call “The Secret”, which I learned a long time ago from watching the Yankees’ Big Three pitchers and seeing how they did it. They all drove off the lower half of the body, using the legs, the hips and the torso in one continuous motion, and the result was a flow of energy from the bottom all the way through the shoulder to the arm. They not only generated more power behind their pitches but also took a lot of pressure off said shoulder and arm so that they threw harder with less effort. (How not to get a sore arm.) I would advise doing some work with the “Hershiser” drill, which aims at getting the hips fully involved—the hips are the connection between the lower and upper halves of the body, and getting this connection will result in a smoother delivery and—yes, increased speed. 8)