This subject strayed a bit off topic for a bit! Regarding the original question:
As a card carrying guy I thought about how I like to see information presented. I would suggest approaching it with humor and enthusiasm. You are not "admitting" anything! You are sharing something private and personal. Not only should he be excited he is reaching the inner sanctum of trust and intimacy, it will give him the perfect opening to reveal anything about himself that he might be shy about or maybe within moments you will be comparing CPAP masks!
As far as how to approach the subject, when things are going well and approaching the runway you might coyly mention you like to leave a little to the imagination when you disrobe. Then reach for a paper bag, pull out the mask while directing a teasing gaze seductively into his eyes, hold them both out (the mask and the bag) and then ask which he would prefer you to wear. There is only one right answer and he had better choose wisely!
If that sounds like too theatrical, you could show him a picture of a
hot babe in a cpap mask and ask him whether you would look as good as her in the same outfit. When you first start dating you are not really meeting the person, you are meeting their representative, so he would not dare tell the truth. Unless it really is the truth, in which case you still have even less to be concerned about.