If you are truely ok with his dressing, let him know you are fine with it. And when he is ready to share more with you, you are ready to listen, learn and participate with him so you both can do it together. You must also figure out what avenues his dressing may take and to what extend he wishes to dress.
What will be acceptable to you? Can you handle him dressed up? Do you want to go out with him dressed provided he is somewhat passable?
You sound very open and accepting of and actually a bit intrigued by it as well. This is all good signs for both of you. Just don't get too pushy and let him come out more to you when he is ready. Maybe you may want to buy him some things to wear and lay them on the bed to open up the conversations between you.
Keep us posted and feel free to ask what ever you need to help you understand better
Rebecca