Hi welcome to Health care magic forum.
Thanks for choosing H.C.M.F.
As you are describing the boy of 13 years is smelling bad, in spite of good washing. 1 st hour he has no smell, but by the end of the school again smell recurs.
The most common cause could be dribbling of urine, that is passing of urine drop by drop, by the time it is observed it is dried up only smell will be there. It usually occurs in case of urinary tract infection
.Urine examination reveals the presence of infection.
Other cause could be the smell of the drugs he is using which are excreted in the sweating he had.
I advise you to consult a physician for diagnosis and treatment.
Wishing for a quick and complete recovery from the problem.