Hi there ~
I understand your concerns. I believe that your son is having a brownish stain on his underwear due to either nocturnal emission
or discharge of semen because of hitting puberty
, or it could be due to incontinence
of the bladder or bowel. Please examine closely since these are two different conditions and the latter needs medical attention. I hope that you consult a qualified doctor if necessary.
Take care and have a lovely day!