My advice is to show him this saidit sub, or the archives from the old itsafetish reddit. Show him pictures of hons in dresses. Tell him that his is what he's going to become, and ask him if that's what he wants to be. Autogynephilia works on the fantasy of being a woman, once you show him that he's just going to look like an ugly guy in a dress and nobody will actually see him as a woman, he'll probably desist.