In the United States, it is customary to tip anywhere between 18-25% for "regular" service in restaurants (though from experience, people - myself included - usually aim for 20%). In extreme cases, tips may be significantly reduced - even absent - or increased; this question does not concern such cases.
My (fairly stubborn) father lives in a country where tips are usually around 10%. The tipping culture there is significantly different: people will usually leave a tip, but may not leave a tip at all for various reasons.
I've explained to him how waiters are often paid less than minimum wage (which is legally allowed in many states), and how most of their income comes from tips. It seems to be impossible to dissuade him from his idea that 20% is an exorbitant amount for a tip - in the past, I've often added to his tip with (my own) cash.
In fact, I nearly consider this to be a question of decency - he's shafting someone (the waiter) who has no hand in what he considers to be the problem (i.e., the tipping culture in the US) and sees no wrong. To me, there is a significant difference between ignorance (e.g., first time in the US and didn't know tipping 20% was the custom) and making a point (which is what he's trying to do and won't accomplish anything, anyway). Refusing to comply (or at least trying to) with local customs is, in my mind, very inconsiderate and extremely disrespectful to those around you.
I've been living in the United States for a few years now and I've grown used to adding a tip to how much I'd expect to pay (which is one of his complaints - he's not used to doing that).
How do I convey to him that regardless of his ideals and opinions on tipping, he is expected to tip waiters more than he likes to?
I hate sitting at a table and having someone cover a check if they're going to leave a terrible tip.