I have found that guessing what my SO, or ANY female I'm with is thinking, is USUALLY a mistake!
If I REALLY want to know, I ASK THEM!

It sounds like you've already done that! He may be either, not feeling like answering, or DOESN'T know HOW TO! Us closet CDs often don't HAVE to deal with these issues. Because we dress alone, in private!

However, I'm NOT going to try and guess his thots and feelings. And I don't think u should try either.

If you're comfortable with this idea, it may help u both!
Invite his fem side over for drinks. Without being confrontational, slowly bring "her" out with questions you've organized in advance. If u can make him/her relax, feelings about why he dresses, and what he enjoys about it, may come out.

One disclaimer: Sex is an important part of MY dressing! I find that embarrassing and would have a hard time discussing that with a GG I cared about! He may feel the same!