Reading this, I get the impression that the question is not as simple as it appears to be. I would inquire as to the reason for the timidity, if such is the case with this pitcher. Usually there are two factors to consider: is the kid afraid of hitting the batter, or of being hit by a line-drive comebacker? The former might well be a control problem, which can be addressed and taken care of. The latter? Has he ever been hit by a comebacker, and how did it affect him? That's a more difficult problem, and there are several ways to deal with it---including, and there's no way of escaping this, PFP, pitcher's fielding practice, which has not been sufficiently addressed in any level of the game other than the major leagues but which should be implemented at all levels. In either case, he should talk to his pitching coach who just might have something up his sleeve besides his arm and who could help. I would not worry about the aggressiveness part of it.