Happy to hear that your dad is on board with the training. That will be the most important thing for him regarding muscle mass and muscle function.
As mentioned in the other thread, I would advise adding a protein shake (2 scoops, ~ 40g protein) every day. In the case of my own father, I pair it with the workout so I can make sure he’s somewhat compliant 
I do not plan on ever giving my dad a protein total or telling him to track his nutrition. I would not advise this for your father either. Instead, I’m starting with 1 protein shake every day. I think that’s sufficient for an otherwise healthy septuagenarian to start.
In the case of your dad, I think a vegan diet has the potential to be very healthy if done correctly. The same cannot be said about many other dietary patterns. Beans certainly count as protein, though we’d still like him to get ~ 160-170 grams of protein per day. I’d bet that habitual intake plus the protein shake gets him there. If some quick back-of-the-envelope math suggests otherwise, I’d then suggest a palm-sized portion of lean protein at each meal, identifying specific foods he already eats/will eat.
Forgive me if this doesn’t directly interact with your question, but I think you may be both underestimating what your dad will and won’t do, as well as putting a lot of much pressure on yourself to absolutely nail your discussions with him. Changing behavior/practices can be difficult, but we can’t really know how difficult until we try and get some feedback. I don’t think that geography (we’re from Missouri) or ADHD have much to do with this. At the end of the day, people are going to require different strategies. It’s difficult to predict an individual’s specific needs ahead of time, you know?