He'll figure it out.
DW and I are on a slightly shorter timetrack than your dad. Speaking as a "dad," I can think of few things scarier than our twenty-something sons deciding that (ignoring travel and associated diving) DW and I needed more excitement in our upcoming retirement than hiking, reading, reading, reading, going to cheap concerts and bars, and putzing around on our property--well, and maybe some more reading. :-)
My guess is that he has things on his mental list; otherwise, why retire? If he gets bored, he'll probably pick up volunteering, some side gigs, or other hobbies. If not, there likely isn't much you can do to change things at this time--and any attempt to change things might not be appreciated. Have you ever asked he and his wife what they plan to do? He might surprise you. (Old reference--look up "Secret Life of Walter Mitty")