What should I call him?

A bit of a strange question. I’ve been dating my SO for a year (and four months), and we decided early on that we would get married once we graduate from college. He’s sincerely proposed to me—without a ring right now—several times, and I’ve said yes every time. My mom has said that an engagement consists of a ring and date, neither of which we have right now. When he mentions me to his mom, he calls me “your daughter-in-law.” (like “Look! It’s your daughter-in-law.”)

What title do I use? To me, it sounds rather informal if I call him “my boyfriend” when I mention him in conversation, and that bothers me since our relationship is far from casual.

Thank you for any input provided! I’m open to advice (just please be nice).