I think your best bet at convincing your dad to go vegan (or at least plant-based) is going at it from a health perspective. At this time in his life, and perhaps for the rest of his life, he may just not care about the lives of animals.

I think to start off is asking your dad if he'd be willing to watch a documentary with you like Forks Over Knives or What the Health, or perhaps he'd also agree to listening to a presentation by someone like Dr. Neal Barnard on the health aspects of going vegan.

See if he'd let you cook a vegan meal for him or two. Maybe make a nice fruit smoothie for him and see if he'd be more willing to eat fruits and vegetables everyday.

If your dad ends up going vegan or plant-based, lets be honest, it's not going to be vegan in one day. It sounds like it's going to be a timely transition. I'd take baby steps with this situation.