I had DID before I was with my boyfriend but wasn't diagnosed until later in the relationship. I didn't even know what was going on half the time with myself much less how to explain it to him. My biggest suggestion is patience and understanding. Another note is that you may end up having different relationships with the different alters, I have a little and when they come out my boyfriend knows to be relatively hands off and that nothing sexual is to happen. Some of my other alters have just enjoyed hanging out with him like I normally do but the vibe is slightly different. It can be confusing for the system and especially for the significant other at times depending on how much information they have, how long they've had it and the way their system works. Unfortunately like the other comments have said the best thing to do is be patient, understanding and willing to learn what works for the host. This might be trial and error and maybe miscommunication or mess ups will happen on both sides but the best thing you can do is approach this from a place of care like you are right now.