I'm not sure about the idea of keeping the bike intact for a tall son or grandson. I know a lot of young people who are kind of into riding, but have no appreciation for vintage bikes. It's likely to be 'just an old bike' that the youngster is supposed to cherish, without any good reason.
It's the OP who clearly has a deep attachment to this bike and it's connection to his father. He doesn't want to get rid of it, through trade or sale. The question seems to be, how to actualize this connection. The bike isn't just the frame. Those are the shifters and brake levers his father handled thousands of times, those are the derailleurs his father fussed over and lubed and adjusted, those are the wheels he trued, and so on. The frame is almost certainly too big, and the handlebars may well be too, but there's a lot of the real connection in all those components. Hang the frame, put the components on another frame that is as close an equivalent as can be found, find the same handlebars in a smaller size (if necessary) and enjoy the reincarnated bike the way it's meant to be enjoyed -- on the road.