Your son seems to have developed a vasculitic skin rash
. It may resemble a bruise
Such lesions can occur over body due to various reasons like intake of various drugs, due to certain food stuffs or due to some infections in the body.
Buttocks and legs are common sites for appearance of such vasculitic lesions.
I would suggest that you should see a nearby dermatologist
soon for its early diagnosis and management.
Hope it .helps
Dr Geetika Paul