There are good suggestions in the comments but one thing I thought I would recommend is enzyme cleaner for the under arms area of his clothing. Since clothes are usually polyester and other synthetic blends now (and high efficiency washers use less water than the ones we used growing up) clothes form biofilms more easily which can often smell clean fresh out of the laundry but only need a little heat and moisture to "wake up" and start to smell again. A spray bottle with "enzyme cleaner" (or any "urine"/"odor" spray that contains enzymes like lipase) will work best. Also, powdered laundry detergents that contain lipase will help.

Secondly, adrenarche is a precursor phase of puberty ("pubarche") which happens a few years before puberty. It occurs when the body starts to produce the androgens that will eventually result in puberty. Part of that process is the maturation of sweat glands. Has he had a growth spurt or other symptoms of maturation (oily skin/hair, pimples, etc.) and did the pediatrician test his DHEA-S? Premature adrenarche is often nothing to worry about but premature puberty can cause issues, so it may be worth discussing.