welcome to healthcaremagic forum
i have seen pics
it is not hives
or allergic reaction
come in mind but biopsy
rule it out
have check his BT,PT,CT,PPT?
hows his mental condition ?
is he okay ?or very angry and isolated ?
it is only on accessible area ,so it may be self induced also
i have come across 2 such case .
so keep watch on him does he induced it or not .
biopsy report also indicate some external factor involvement .
so wear him full cloths and keep him along you .
otherwise i dont think it is serious matter.
but without examination i cant tell it finally.
i hope i have answered your question
dr gaurav patel ,ahmedabad