The most direct way to go about this is rent the 1991 Denzel vehicle, Mississippi Masala. As we all know, it explores the interracial romance between an African American and Indian American. As soon as you press "PLAY" study your man. Take note of his facial expression and body language. Are his lips taut with tension? Can you read concern about his forehead? Are his eyes wide and empathic? Is he shaking his head and sucking his teeth whenever Mina cries?
Now bring your documented findings to the most senior rabbi in your community. If you can't locate him the senior lifeguard at any JCC will also do. Ask him to read these "tea leaves" as it were, and find out if your man is, as you put it "open to a serious relationship with a Non-Jew."
If for any reason this doesn't work, there are other tools at your disposal. Broach the topic with him. See where he stands on the issue. It's a complex thing for many Jews... any minority. Heck, anyone on the planet who feels passionate about preserving and continuing their cultural/genetic footprint.
Make him think long and hard because he may have a knee-jerk reaction that isn't even his own. Maybe it's his parents' expectations? Maybe he thinks he's more carefree about it then he actually is? Really press him on the issue, and both of you explore it in all its angles. Envision family mingling, holidays, Christmas trees, weddings, baby names, Christenings, circumcisions, Bar Mitzvahs, church. You don't want to feel like a second-class citizen, being "punished" or "stifled" for your non-jewishness. Your future together can definitely work. Tens of thousands of couples make it work every year. Just be open. I wish you two much Mazel when you find yourself having this conversation.