As a coach myself, I have to agree with his coaches in wanting him to use the whole field. I agree with only swinging at strikes, however he should be driving the ball into the oppo gap on fastballs on the outer half of the plate.
If he can hit anyone's fastball, he shouldn't get too picky with fastballs early in counts that are hittable, oftentimes those are gonna be the best pitches he gets in an at-bat. Since he's playing college, I would imagine that teams are starting to "scout him" and the book is to get him to try to pull outside fastballs (often resulting in ground-outs) or strike him out with offspeed. He has to adjust, otherwise that is all he is ever going to get (except for occasionally pitchers making mistakes).
Baseball is a continual game of adjustments for hitters, especially at levels above high school. If he shows that he will drive outside fastballs oppo, and lay off the junk that isn't a strike, he will be more consistent as a hitter and will get better pitches to hit.