It's hard for anyone else to guess what your father would like. You know his tastes better than we do. But I am sort of inclined towards Sho-Tunes-R-Us's view that if, in his 70s, he wants to see a Broadway show with you for the first time, give him a big, splashy Broadway musical. Many of the musicals - Phantom, Chicago, Moulin Rouge, Lion King, Aladdin, even Waitress - have film adaptations or started as films. Did he like any of them? (If My Fair Lady weren't closing this weekend, I would suggest that.)
Ain't Too Proud is a standard jukebox musical but it is well-performed and choreographed. If he likes the Temptations, that's a good start. At least he would enjoy the score.
I haven't seen Moulin Rouge! but it certainly counts as a big, splashy Broadway musical. Would he enjoy something like that, or would it be too much?
Come From Away is probably a pretty safe choice, although some of things that impress frequent theatergoers - its minimalism and sharp direction - might seem underwhelming to someone expecting something splashier.
I assume you have ruled it out for your father, but Hadestown doesn't seem like a bad choice either.