My 9 year old husky boy. I originally ordered a size 10 knowing I might have to exchange it. I did and bought a size 12 instead. Fits great. Covers the stomach when raising the arms and relaxes without tightening the chest area when lowering the arms. He really likes the Dri-Fit material. If your son is of average height, 10 is probably perfect for a 9-10 year old. If he is a husky I would go one size bigger than his age. If it makes sense The exchange process was easy. There is a return label just in case.
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