I would be curious to hear how tall the player is and how tall he is projected to be. Some things to consider...
Glove length is measured from heel to far pocket edge, including the curve/depth of pocket. So, as a result, not all gloves of a certain measurement are equal. As example, those with deeper pockets will be, in effect, shorter.
Generally, a player who is asked to play all three of those positions is typically afforded a little leeway in glove length ( a 3b can use a slightly longer glove with deeper pocket because quick transfer is not as frequently necessary. There is often more use of the pocket when fielding as opposed to more used of the hands because 3b is closer to the hitter and more subject to hard pull hits and thus more reactionary than read and attack). So, as long as 3b is still in the mix, he may not really need to change at all.
So, why, specifically, are they suggesting a shorter glove? Is he not using his hands enough when playing MIF? does he play more MIF than 3B? Is the pocket too deep on his current glove? If those are the reasons, he should try to find a good 11 3/4" glove with a more shallow pocket. There are many very good brands with high end series that will be sufficient. He just needs to try them on until he finds the one that is comfortable to him. Depending on the problem, it may also require that he alter his fielding technique to more use of the hands.
Also, depending on the reason, the solution might be as simple as having laces tightened or loosened to acheive the desired affect.