Your question is very challenging. You didn't mention whether he is otherwise healthy. If he is an otherwise healthy young man, he probbably has colonization with bacteria on the skin that are leading to the smell when he sweats. You can try using a topical anti-bacterial such as Hibiclens. Check with your doctor or a pharmacist to discuss any particulars about your son's case to make sure that it is safe and to review risks and precautions for use of this type of products. Several applications are usually sufficient to take care of colonizations.
I hope this helps. Again, you may want to discuss further with your pediatrician if this measure fails to work or if your son has other unusual symptoms. Rare inborn errors of metabolism can be associated with unusual body smells as well as other health issues in children
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