Labels are for description, so figure out what you're describing and then give it a name that you're comfortable with, rather than picking a name and then trying to live up to it.
If you're feeling fizzy feelings, just say to him -- "I was wondering if you feel like we do now, or might ever, have a 'relationship'... something that's less than a full partnership but more than just casual, that you might call boyfriend/girlfriend or secondaries. Personally, I'm not sure, I know I like you a lot but I think we could go in more than one direction and I'm curious how you feel."
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.