Smaller than other children

My son is smaller than the other children of his age group. Though the doctor has said that there is nothing wrong with his growth the difference is prominent. Last month it was sports day in their school and they were all given special t shirts with the school logo printed in them via dye sublimation. The t-shirts were of a average size to suit 4 year old but it was big for him and they did not have a smaller one. He was very upset and did not go for the event. I am very worried about him now. What should I do? I am at a loss.