Trust that he’ll find something to do. If he worked as hard as you say, he will continue to keep himself busy, hopefully at something that he finds challenging but do-able.
When my father retired, I think he actually worked harder than during the 40+ years he worked at a local dairy plant. He not only did chores on his own small farm (a couple horses, ducks, chickens, etc.), he would ride horse (or cart) over to my brother’s dairy farm to help there (he also had an ulterior motive in that his youngest granddaughter was born after he retired). Of course, he was never on time for anything because he was always willing to stop and chat, to lend a hand, or to share homemade wine, with a neighbor.
His “thing” was chainsaws. He’d buy them used, at estate sales, repair them and give them away to family and neighbors, often right when they needed them. (We suspect he kept one hidden in the horse cart.) When he passed, he still had four or so under repair in his workshop.
I suspect that your father will react to retirement in much the same manner. Give him space and support him while he figures out what he wants to do with the next phase of his life.