Armpit odour in a child

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Hi, my son is 3 years old and if we don’t put deodorant on him his armpits will get really smelly - like an adult. He wears cotton clothes and I change and wash his clothes daily. We don’t eat foot with spices or too much garlic / onion. We took him to the Dr who sent a urine test and all results were ok. We have tried both baking soda and a natural deodorant and both work well but without this, the smell comes back. Is there something we can do? It’s obviously not ideal for a 3 year old to have to use deodorant.