First off, is he really just a crossdresser or is he repressing his gender identity? Even if he is fully convinced that his gender identity is male now, he may discover that his gender identity actually isn’t male in the future.
(It will not be his fault, as these issues really do take a lot of time to resolve.)
For every story that the husband transitions after marriage and they happily continue their relationship as two lesbians; there is another story where they get divorced. Do you love this person so much that this person’s gender identity does not matter? Are you willing to continue loving this person if this person is now female?
Can you imagine doing this to your boyfriend?
Assuming you are willing to continue on a relationship with your crossdressing boyfriend, regardless of what the future may bring, a few ways to handle it are:
#1. Encourage him. If he loves lingerie: surprise him with some, take him shopping for some or include them into your makeout sessions. If he loves getting dolled up: take him out to a romantic dinner while crossdressed, go to a spa together or get married in matching wedding gowns.
Don’t worry that this may push him over to the trangender side of the umbrella - chances are that if he is going to turn out to be transgender, it won’t be due to your encouragement.
#2. Ignore his crossdressing activities. Don’t talk about it and just do nothing if you happen to see evidence of whatever he’s been doing. Don’t ask, don’t tell in the armed forces was a terrible policy; and it will be terrible policy if you do this. By not actively engaging him, you may not know what is going on until he suddenly drops terrible (but not unexpected) news on you one day.
#3. Actively discourage him. The desire to crossdress is something that cannot be reliably extinguished. Discouraging him may make him suppress his desires - which will almost definitely bubble again to the surface. This may also make him resent you - and this may be the end of the relationship.
Whatever you decide to do, continuing a relationship with a boyfriend who crossdresses brings greater risks that somebody will get hurt (hence my advice to be careful about the whole thing); but there will be greater rewards (such as a man who understands why it takes so long to get ready, a man in touch with his feminine side, etc).
But if you truly love this person and want to spend the rest of your life with this person - the crossdressing part shouldn’t even matter.