First, for some bizarre reason, his uniform had a belt!
Secondly, his side elastic waistbands were much smaller than everybody else's - perhaps ½", ⅝", or ¾" at most, as opposed to the standard 2" waistbands.
So who knows? Maybe the belt was there solely to cover up the abnormal, undersized waistband.
Or maybe The Powers That Be were trying to somehow give each character their own distinct "look" in the movie - Picard with his belt, Riker with his sleeves rolled up, Beverly with her "Doctor coat," Worf with his baldric, Troi in the TNG uniform ... showing us two characters in the different uniforms at every opportunity (Geordi and Data on the station, Riker and Worf in engineering, Picard and Kirk doing their thing, Picard and Riker at the end, etc.).