I think maybe the most important thing is what I would NOT do. I would not try to be whatever or whoever I thought he wanted me to be. I would simply be myself, because I can't change that and you can't fake it for a lifetime, can you?
I would invite him to go somewhere fun, but quiet. Maybe to a lake or an ocean somewhere. A place where there was a lot to see and do, but also a place of reflection. When the time was right, probably a few hours into the trip, I would simply and lovingly tell him why I thought he was the most amazing man on the planet. I would then let it soak in and just enjoy the rest of the time with him. No questions. No pressure.