I agree with both of the previous posts. some things are fantasies and fantasies only and (best left that way) You should certaily talk to him and see if it is really for himself.
The other option is he might be a crossdresser himself. I am and I am married and have been with my wife for over 11 years (six of those married and still going)
If you want to be sneaky about it ask him if you can give him a makeover, if he says yes, ask him to wear you underwear, then some clothes, if he is letting you go further and further than you might want to make sure he has not been going through your things already ; P
Just another thought if he is a crossdresser ask yourself if you think it is wrong for him to dress or do you think it is wrong because "society" thinks it was worng. Not too long ago women were shunned for wearing pants. Think about it.
Good luck!
