I would also take that approach... not that you think they are negligent in not seeking more attention for it, but that you just need the verification that it's nothing contagious.
Has he had any allergy testing? This kind of insanely dry skin/excema, etc usually comes hand in hand with severe allergies, usually to something dietary. I would suggest to mom and dad to get him tested. The testing method now is MUCH better than even a few years ago.