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The scales n the scrotum
you were referring to are nothing but the layers of dried dead tissue( skin). It is a normal process and you need not worry about them. If it is really troublesome using lactocalamine preparations on the dry area will relieve it.
Coming to the stinging feel when washed with water- it is common when water falls on the wound. More over your son had gangrene
. During the surgery the skin will be removed leaving the underlying muscle ( Dartos muscle) and the sub cutaneous tissue open which is sensitive to touch and temperature. He should be fine once the wound heals completely.
Wish your son a speedy recovery.