A child born into the Royal Family is going to spend their entire life adhering to strict codes of conduct, dress and behavior, so it makes sense to get them used to it from a young age! Prince George is growing up before our eyes. He's no longer a toddler, and so he has to dress to match his office; even if he may not yet understand why.
If you see a picture of the young Prince out and about, you'll notice he's always wearing shorts, rather than pants This is a Royal tradition going back to at least the 16th Century, when Royal boys used to wear either breeches or dresses until they were eight years old. Formality has relaxed enough to allow George to wear his shorts, but not so far to allow for pants just yet. You'll also never see him wearing a t-shirt with his favorite children's TV character on it!