He's got all the makings of a very entertaining grandpa.
There's always a risk involved when you secretly film people finding out big news. Maybe they won't be as excited as you hope that you're proposing or back from Afghanistan or ordered Domino's.
But this dude (and his wife, too, for that matter) had the perfect reaction to finding out he was going to be a grandpa. At first, it seems like he might not quite understand what he's being told. But then it turns out his excitement is so overwhelming that the only way he can express it is with a private jig.
Just don't let the baby see you do that dance, or you're going to be doing it over and over and over again.