Once again, I am not a mental health professional, nor a self defense professional for the record, and @Dr.Drake makes a good suggestion to involve them.
@VOW is very possibly correct that getting out and away is the best answer, but for many people it isn’t financially viable, and I don’t know your options, so that is best for your to evaluate, but it’s almost certainly the lowest risk of harm to you.
Having said all that, get other people involved. If he’s attempting to convince others that you’re trying to poison them (which seems likely to rebound on his believability, because, well, duh), you need to make sure your mutual contacts are in the know of his issues.
Second, break away as much as possible from interactions. If you can, do it in a positive manner - say you respect his concerns, and as to minimize them, you’re going to keep your distance, and let him manage his own food away from your interference, as much as possible in shared living situation. Absolutely work to get out of that location and away from them as quickly as possible (again as life and finances allow), but reduce the contact and possible points of conflict as much as possible in the meantime.
To add additional protection from spurious claims, I’d suggest setting up an unobtrusive home security / web / spy cam. If you don’t have the cash to buy one, you can possibly repurpose an old cell phone in you have an older spare lying around.
If your property allows it, I would also strongly consider adding a screw on security latch as a cheap protective option for your own room, if you have a private bedroom. Something as simple as an $8 stamped steel option won’t stop them, but might buy you some time to get out a window or get help.
Lastly, you mention a large knife collection. If he carries any of them around, rather than just admire them in the residence, there are jurisdictions that make that a crime, which can be actionable. Here’s a link to each state where you can consult said laws.
On a similar note, if you are all renting a property, many (by no means all) prohibit the possession of weapons, often including non-kitchen knives on the property. It, like the link above, is a longshot, but check your formal rental agreement if any, as another way to involve other vested parties in shooing this individual out of your life ASAP.
But in all honestly, if he believes you’re trying to poison him, I do agree he’s far to unstable to expect any subtle or common sense options to work as more than the most mild deterrent. If you can’t leave, and they start threatening you, get the hell out even if you have to couch surf for a day or so, and call the local law enforcement about your concerns. If your webcam / security cam recorded it, all the better.
