I think what's happening is that he remember having chicken pox, and now that he is older, he assumed it is shingles or chickenpox. He just simply asked mother how to deal with chickenpox or itchy rash, and she trust her son whatever he says (as if he already been seen by a doctor or something) and told him to take an oatmeal bath for relief. Sexual diseases are usually far from a parent's mind especially when they don't think their child will share it with them.
Guys do tend to call their parents for suggestions. My husband does.