As long as you know how much and what you are eating, it's a good idea to leave your dad's eating habits alone. You don't have to eat what he eats. He doesn't have to eat what you eat. It can easily turn into a war of 'What I eat is better than what you eat...' and that turns nasty fast!
You say he gets mad when you walk away from him when he's making bad food choices. He may be interpreting that as some sort of comment on his eating habits. Just stay away from him when food is involved, if you can - eat your own meals separately or out of the house. If you can't stay away, then try to be calm enough to not let him goad you ('Have some.' 'No thanks, I'm not hungry.' 'What's wrong? Are you sick?' 'No, I already had something to eat and I'm not hungry....' and so on) because food choice and preparation can become a battlefield! The thing is: it takes two to to begin the battle.