It is my assessment that the sore on the head are due to the disease process it self and not due to penicillin.But the exact diagnosis can be ascertained after examining the lesion. Penicillin allergy
will manifest with rashes...and when severe willl cause blisters...and your child will be very sick...
Apply some topical antibiotics to that area.if it does not heal consult your local physician.
i hope he gets well soon.have a nice day