As little as possible, he's nuts. So was mine so I know how you feel.
So many people believe this. Is he listening to info wars and watching you tube. The Sumerians wrote the history of the world that all religions stole from. That is how we get Genesis. The Jews took the female goddess out of the Bible because it was a god and goddess that created man, it took many tries until a race could mine for gold and be slaves. The flood was also in the Sumerian myths with a different name than Moses. A god was tired of humans and a nicer god told a Moses type character to build a boat a save two of every animal. When they landed they built a burnt offering. The entire beginning of Genesis is Sumerian myth. The gods mating with females and the Annunaki as the watchers. Will he go to Wikipedia. Look up Enki, Enlil, Thoth and Sumerian myths. Good Luck, my dad became a right wing crazy because he was dying with cancer and listened to the crazy radio programs and he was saying politicians should be hung from the lightpoles. This was not my Dad so I understand your concern. Of course, I am condensing the Sumerian stories but maybe it would help if you could find something for him to read.
I dealt with this in my ex, I convinced her to seek a mental health professional and she is still seeing one. That kind of paranoia is really unhealthy, if you care for this person you need to somehow get them to see someone.
Why do you love your dad? I mean to say, if he wasn't your dad but still was the exact same person, would you love him and want to be around him? He seems to be a condescending person who won't even listen to what their son has to say nor try to understand. Sometimes parents are nothing more than two people who had sex and popped out a child (or adopted), and you don't need to feel that there needs to be more (but we're indoctrinated to think otherwise). You've planted the seed, there's not much more you can do unless you're willing to spend more time on it (keep in mind it could be vaine), which is totally up to you. See if you're better off continuing to try to dissuade him or to go plant other seeds elsewhere (or to do both). Lost causes do exist, so be careful not to screw up your mind over this. I have a slightly similar situation with me being a vegan and my parents being irrational about it (saying every point I make is irrelevant or a lie or that "science is just science"), I bring it up whenever I'm with them (which is quite rare because they are acting immorally and I won't stand for it) and long for the day I can completely stop seeing them.
Tell him what you told us, that you love him but you are concerned about his beliefs, that it bothers you that he won't be open-minded, that he should know better, etc.
I think you underestimate the extent to which belief, when one gets deeply immersed in them, whether it is in conspiracy theories, ideologies or religion, get embedded inside your entire way of thinking, shaping the way you think about (almost) everything, makes you seek for constant confirmation, especially when confronted with conflicting information in order not to have to actually deal with it.
When a victim once sufficiently indoctrinated meets disbelief or perceives a lack of reinforcement ("open mindedness as it is usually designated") of those ideas from their immediate surrounding, be it family, relatives or friends he or she usually has by then so thoroughly identified with this particular belief or set of beliefs as to take this as an attack on their person as opposed to on their ideas and hence see this as betrayal. And I mean really no less than that.
This is what makes it so difficult to deal with: when it reaches this emotional level when rational arguments no longer have any effect.
This applies also to veganism for example. Something in which you probably already have ideologically immersed yourself and likely are able to see only the side of it, using scientific studies, or using carefully concocted arguments about morality or otherwise, confirming your already held beliefs. It might come to a point that it is getting extremely hard for you to imagine other people can, based on honest evaluation of all information they have had access to come to other conclusions than you.
To Matthew I don't think you can do that much about it. I would think that attacking the irrational (set of) beliefs head on is usually bound to fail for said reason. A devious Socratic approach or perhaps even better as you did with your example to show how to derive the gravitation of Jupiter from simple Newtonian mechanics, embedding the problem in related science and getting him to properly understand the underlying science is probably on longer term more effective. Let him mull over it. Likewise I think, it might work over the longer term, to make them familiar with critical thinking, learn to recognize all the fallacious traps, and apply it first to subjects they are still/ already skeptical about, since other peoples mistakes are so much easier to recognize, and then expand from there.
I wouldn't say that we are indoctrinated to think our parents are more than two people who had sex and out popped a baby. If our parents were to turn their backs on us immediately after birth that may be the case. Parents do a lot more than just give birth to their offspring. They nurture and care for us. Truly most of us wouldn't have survived were it not for our parents so reducing them to nothing more than a vehicle to bring us into the world is pretty damn ignorant.
Get him to read Carl Sagan's book "The Demon-Haunted World: Science as a Candle in the Dark".
The book is intended to explain the scientific method to laypeople, and to encourage people to learn critical or skeptical thinking. It explains methods to help distinguish between ideas that are considered valid science, and ideas that can be considered pseudoscience. Sagan states that when new ideas are offered for consideration, they should be tested by means of skeptical thinking, and should stand up to rigorous questioning.
I would go with.michael shermer's talk about the baloney detection kit. (science)
Tell him you are part of "them" and send to watch him. In other words: massive amounts of trolling possible.