My bf and I had two dinners that were pretty quiet/romantic that we quite enjoyed.
The first was dinner at Tokyo Dining in EPCOT. I scheduled our ADR for 6:50pm so that we could catch the last Masturiza show before dinner, and so that the sunset was right around the time of our reservation. We were seated at the window that overlooked the lake and Spaceship Earth. It was really beautiful.
The second one was dinner at Hollywood Brown Derby in DHS. We had a late reservation, so we wound up missing Fantasmic!, but it was worth it. This was probably our favorite meal of the trip in terms of food quality and quiet atmosphere.
I was originally going to suggest dinner at Be Our Guest, but since it's so popular right now, it's really loud during mealtimes and getting an ADR can be a pain in the butt. Plus, like you said, your bf won't be in MK for more than a couple hours, so you probably want to spend that time exploring the park, not eating a meal.