a brief description in really not sufficient. a medical provider would need to see this skin eruption to make an accurate diagnosis. many skin lesions can mimic chicken pox but of course chicken pox does not just involve one thigh. there are many things it could be from a localized infection like bullous impetigo to shingles or a retained foreign body so please see his medical provider so the mystery can be solved.
my son has a rash that looks like your chicken pox slide but it's only on his thigh and it's been there for over a week. there is one very large blister that is red all around it. the rest are small most with whiteheads.