Really good comments and advice from all. Lets go back to what Fungo said. Do not let the draft dictate what the pitchers arsenal will be
Now about pitching.
If your kid can throw both a 2 and 4 seam FB and can control it, then each pitch has a place and a situation.
If the 2 seamer has any sink at all, its a great pitch when behind in the count and can induce ground balls. 4 seamer with some movement on the plane can be very helpful in situations where a K or pop up would be ideal.
Control of the pitches is paramount.
Why not have a 4 pitch arsenal if he can throw it.
But, do nothing else unless its to improve the pitchers game plan and the chance for team success.
If he gets'em out, and throws 90ish, the scouts will notice.