I know for a fact I would have to catch my breath. I think about him sometimes, usually when a song comes on the radio or I watch a movie from the time we were together. My stomach would probably do a summer salt. I would ask him how he is, and ask him to come up to my apartment for coffee.
I'd be very nice, we broke up my senior year of high school, we were both kids, and we broke up under very stressful circumstances. I know we would have to talk about the old days, because that is what you do.
I'd ask him what brought him to my door, and hopefully it is just a friendly visit. I'd invite him to stay for supper and meet my partner.
I'm pretty sure things wouldn't be that weird unless he told me he wanted to get back together again. Then I would have to ask him to leave.