My ds is 13 weeks off turning 4. He still wears size 1 with the odd 2. He started pre prep last week and the smallest uniform was a size 4. The top came down past his knees. I have adjusted them to fit. We had him investigated because of his size (FTT) and because he had a fits. It turns out that is just who he is. If worried ask your gp to refer you to the pead at your local hospital for a full check over. I might just give you peace of mind.