welcome to hcm.
though your question is very brief yet with those information i will try to help you.
brown spot over shins may be due to thyroid
disorder/erythema ab igne
these are common diagnosis.
does he stays longer in front of heat?
it may be related to erythema ab igne.
if there is itching that may be prurigo.
,sweating may be due to thyroid problem.
diagnosis can be made after detail history,complete examination and investigation.
no need to worry.
effective treatment is available for each diagnosis.
please consult a dermatologist
hope this will help.