Whatever they want to be referred to as. with some people it varies dependeding on the time/situation (ie. genderfluid people), or they want gender neutral pronouns, like they/them. I would go with they/them for a genderqueer person until you have the chance to ask or they inform you otherwise.
My boyfriend calls me "datefriend". It's a solid gender neutral term for it. But it all comes down to your s/o's preference.
I've heard of partner, significant other, enbyfriend (enby = NB = nonbinary), datefriend, and datemate. Some genderqueer people are also fine with being referred to in gendered terms, at least some of the time.
My last paramour was genderfluid. He was a female-bodied person who did on occasion was a girl but the vast majority of the time, he indentified as a guy. So for convenience sake, we've gone with the masculine terms. ---------- Post added 4th Jan 2017 at 08:57 PM ---------- Oh! But I would like to add that this is just what he liked. It's different from person to person so you might just want to ask if you're dating a nonbinary person.