George doesn't look v tidily put together & I'm not overly fond of the piped shirt but it's not that bad. If you dissect the outfit it makes sense as a choice.
I think he's in trousers now because he's about 6.5 yo and getting quite tall! He's too young for them to put him in a suit, I think, and I can't see them letting him wear a shirt & tie with no jacket. I know Kate has a bit of a thing for piped shirts anyway but if he had a plain white shirt on for the photo he'd have no colour next to his face - not ideal for photos. Piped edges do the trick. Personally I'd have put him in a v neck jumper but perhaps that looked a bit school uniform-y?
100 years ago he'd have been in a short-suit with eton collar type affair or probably a sailor suit. I'm surprised Kate hasn't pushed a sailor suit on one of the boys actually!