Hi Emma, can I start by paraphrasing a 30 yr old quote. 'That's one small step for you, one giant leap for your mind'. I can only applaude your honesty and openess in your post.
I can only imagine how it must have felt seeing your SO in your clothes etc, a total betrayal of your own femininity. He probably used them because he was unable to buy and hide a wardrobe of his own.
You had questions of him, was he gay?, did he want to be a woman? Without wanting to worry you, of course this is a remote possibility, however, the chances are that he is not and does not. By and large the main % of cd'ers are just that, content to dress and look like their alter-ego and are heterosexual.
From your post it would appear that you have been together for some time, and has been 'the man' for you; he still is that man just because he chooses to dress differently, in private, to 'normal' men.
Why did he hide it? Fear is the main reason, fear of being ridiculed by his 'macho' peers, fear of losing you.
Work with him slowly, set your own rules and pace and I hope that in time you will see him for what he is, (we tend to be more caring, re our spouses/so's than a lot of masculine men).
Once you have 10 posts you can apply to join the FAB (Female at Birth) forum where the r real girls on here will give you lots of info and not be biased like we cd'ers can be for obvious reasons.
