Yeah ok, after the additional info posted, Imma say it: this guy is too far gone to argue with. He might get back to the headspace where you can argue with him, but he aint there atm. So engaging him in his favourite topics is not going to do anything productive.
What he needs is a job. Seriously. I'mma be incredibly Typist and say: INTJs that do not have something to do, something to conquer, something to control and direct - like a job - tend to start taking over the world in other ways. I kid you not. He *needs* a place where he can flex that Ni and Te in a constructive way, instead of letting it cannibalise his mind already. He is like a stallion, stuck in a stable 24/7, instead of out in the meadow where he can run. It *will* drive them bonkers.
So, for you personally, I'd let him ramble about his thoughts - let him bleed it off somewhere - without engaging him. Be accepting of his pov, without validating it, agreeing with it, arguing it, whatever. Let him tell you his side of the story and be understanding as such. Just the fact that you'll let him vent, without anything else, and still want to be around him and *do* stuff with him will do a great deal already for him. He'll feel heard, while getting the contact and friendship he needs. It ll take away a lot of pain and self-doubt, though not all.
Secondly, if you can, find him a way to get back in touch with reality. Preferably through a job that he can feel valuable in, something that is suited to his skill set without his disability getting in the way. Otherwise, a project of some sort where his particular skill set would yield tangible results for him to see, measure and improve on. This will do what [MENTION=8936]highlander[/MENTION] stated before. It'll get him back flexing his Te and get him back in touch with reality. Also, this stage will build up his confidence again. His faith in his Ni, backed by Te. And it will make him a lot less grumpy, defensive, wounded and frustrated, which will lead to less conspiracy theorising. (believe it or not, I do this with cats and play time when they bully others

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Third, if you again can muster the energy or resources: get him out of the house. Like yourself, he is Se-last. That thing has been stabled wayyyyy too long and it is my experience that occasionally INxJs need to be able to go bonkers. Mine does so with drinking- and turning himself into an ESTP moron in the process-, buying yuppie clothes, having the urge to go *do* something completely non-sensical like eating the weirdest, grossest food he can find while going in to town, or even just laughing at the most slap-stick style humour he can find on youtube - I swear I do not recognise that man when he gets that way
But it will make him in the moment again, forget all his angst for a second and be a kid again - without worries. It'll clear his head, hopefully and recharge his batteries. Like the restart button on a computer, it should hopefully clear out any bugs in the system
At the same time, that should provide some temporary relief along with the acceptance you give him (FiSe). Most important though is to get that NiTe back to work, and get it to do something, coz otherwise evil overlord plans will be made
