Rashes that come on suddenly and cover a wide area are often not very serious at all. They could be a fungal infection--not serious and easy to treat. They could be a virus, that will go away on its own. They could be a form of eczema, which needs just a cream. Why not have him see a doctor to find out?
Take care.
Dr. Rockoff
I second what the doctor says. What's embarrassing about brown spots on the body? Gee whiz...