You can lead a horse to water but it won't make him drink. I think it's kind of fitting here. All you can do is educate your dad on the ingredients in Cola and tell him what those ingredients do to the body. Ask him not to quit, but maybe just to slow down. Tell him to treat every meal like he's in a restaurant, that he should drink his soda and his water. You might also have him try decaffeinated soda (buy him a weeks worth) and see if he likes it. You could also try having him try another beverage at least once a week, he may find something else he likes that is actually better for him.

Good luck.