BO in a child of this age is not entirely unusual, although there could be additional causes rather than just putting it down to him being a sweaty smelling beast of a boy!
There may be a slight deficiency of the enzyme that breaks down certain ingredients of normal foodstuffs. Without these enzymes chemicals accumulate in the body only to be excreted through the glands in the skin producing a noticeable odour. A well balanced diet and avoiding certain foods can solve the problem or at least alleviate them to some degree.
Certain synthetic materials in clothing can also react with sweat and cause an odour in their own right. Additionally, some people are simply born with more sweat glands than others and need to take extra precaution for deodorising throughout life.
You could try Anhydrol Forte (used during the night and washed off in morning) to see if it helps, but please check the ingredients to see if your son might have any reaction to them. Ask the advice of the pharmacist to be sure.