I would tell him it was OK and also direct him to this forum, in the spirit of meeting folks who are able to navigate the CDing successfully in their marriages/relationships.
I have tons of reading material on the matter and I would ask if he was interested in reading that as well. I would explain to my son why I know so much (with my SO's permission), and encourage my son to talk to me freely about it.
I doubt that he'd like his mother's advice on makeup or clothing, but if he asked I would give him pointers on how to present as realistically as possible.
If he wanted help with frilly dresses and sissy maid's outfits, I would explain the difference between having gender identity issues and fetish, and advise him to go online and find suitable resources for that. I couldn't possibly help my son engage in fetish dressing.
