Yep, did this when my dad was 76. I mentioned that every cloud has a silver lining and both of them have a chance of another silver lining (I'm assuming the wee wifey-to-be is also getting on).
You could mention that your memories will always be there but that people also need companionship.
On a lighter note, try saying that nowadays they could just shack up together for a few years to get to know each other, but make a joke of it.
Above all be glad for them and wish them well. That'll be important to them.
Then take him aside, feed him a drink or two and talk about the will!!