Hey it is good news if his 2-seam fastball is moving consistantly. When you say it is moving outside you might want to clarify, outside to a right handed hitter or outside to a left handed hitter? Also, does you son throw right handed or lefthanded?
In any case, the adjustment is simple, change where he is aiming. You might think of the path of his pitches as a Jets' smoke trail. If his pitches move in one direction he just needs to choose his target, where he wants the pitch to end up, and imagine a jet trail from his release point, moving the direction his pitches move and ending at his target. You can make it even simpler, if his pitches run "outside", aim down the middle when he wants to throw outside, aim inside when he wants to throw down the middle. He must have already made the adjustment when throwing at 60'. It is a slightly different adjustment at 46', but he should be able to make it easly and take advantage of the movement on this 2-seam.
Good luck to you and your son in the thanksgiving tournament.