It is a generational thing. My mom and in-laws are the same way. They can't comprehend when my family takes them out for lunch and I leave a $30-$40 tip on $150 tab for example. They always give me the "Wow, that tip could have paid for lunch for days for us." It is just old people using their past experiences to influence their current behavior.
You don't need to talk to your dad--he paid for your lunch and thinks he did a good thing for you. You made the situation right--end of story. Lecturing him, will just make him feel bad/angry and frankly make you look ungrateful for him doing something nice for you.
Now you know, in the future when you go out with him and there is a chance of him paying, bring an extra 20 spot and tell him you want to get the tip. This is exactly what I do with my wife's parents, who are notorious at calculating the tip to 10% and making sure they leave the bill at the exact dollar amount ie: the bill is $15.45--the waitperson is getting a $1.55 tip.