I am currently taking the AD that they use to help when they are trying to stop smoking. It is Welbutrin or call Zyban when they give it for smoking cessation. I like Welbutrin, it is weight neutral (doesn't cause the weight gain that others do) and I had no side effects. Of course I am not sure about the sexual difficulty that some of them cause men.
I am sorry but I had a good chuckle thinking about your H having that difficulty in front of his much younger OW. I think it would be funny if it happened to H with his OW. At his ge, it is possible.
Everything happens for a reason, maybe Dad needs to find that it isn't better out there, he needs to realize how good he had it here. Maybe he will find God and that is the most important thing when he finds Him he will know he is supposed to come home.