I'd take it to an airbrush shop (like one that makes signs for businesses, sprays storefronts, etc), and have them do a custom design on it. For instance, if he really likes a particular theme (fantasy, science fiction, hot girls, etc) you could have them go with that motif. A nice white chassis like that would be the perfect palette. Just make sure to mask it off in a way that paint won't get on the components...