I'm going to assume you know he'll say 'yes'. I say this because I had a proposal once in a crowded restaurant where the guy got on his knees, and the whole restaurant turned to watch. Of course I said yes but two weeks later we called it off. So, yeah, be sure if it's public that his 'yes' is for you and not to avoid embarrassment or an awkward situation.
In front of the castle or the Disney statue would be nice if you do rope drop or EMH (morning) and can get it to yourselves. Unfortunately, it's often very crowded later in the day.
There's also a spot on the bridge from Tomorrowland to the center where you'll often see Photopass photographers. It's got a little trellis and is just off the bridge with the castle behind it. That would be a wonderful spot.
If you are more rustic people, you might go over to Tom Sawyer's Island. Or how about in the Swiss Family Robinson tree house, again for people who like rustic. Don't go in the height of the crowds, though or you'll be pushed to move along through it.
Don't do it on a ride, as you'll miss out on the ride that way, but you could do it just after or before, if you're so inclined. I'd pick Small World, if that's the case.
Also, there are spots in the back of the new Fantasyland where you might have a view of the Prince's castle. That might work.
All the best. I hope you find your perfect moment.