Have him wipe neatsfoot oil on it, then play a lot of catch.
Seriously-- it's like any other skill rating (as opposed to a tool rating like range, speed, power) in that it develops during the season dependent on playing time and coaching. The key coach for glove development throughout the system is your ML fielding coach; but the fielding rating of your bench coach at the appropriate level may have some role. It has been said by sitestaff that the position you play the guy at matters, but I'm not sure I believe that, would welcome others' views.