He's hoarding weapons and has set himself up in an echo chamber, presumably with likeminded people online, so he wont to sense check any information he's getting - he will just be working on confirmation bias now so he will spiral into these views becoming more entrenched.
If he was hoarding weapons for any other reason, would you feel comfortable staying with him? I absolutely wouldn't. It would be madness.
Try to read your words back, objectively, and imagine you had a daughter or niece saying this about their partner:
He's bought thousands of pounds worth of equipment - including a machete, an axe, a crossbow and a pellet gun. If he could buy a real gun legally in England he would. I wouldn't be surprised if he bought one illegally.
What would you tell her to do? Do that.
If he's having a mental health crisis then yes he would benefit from professional support but that isn't your job, it's vanishingly unlikely he will engage willingly and you shouldn't have to do anything at the expense of your own safety.
A house where there are weapons and someone who isn't behaving rationally is not a safe place to be.
Women are not rehabilitation centres for men and you need to put your own life jacket on first.
I would leave, be somewhere safe and if you think you need to, call his GP surgery or the police for a welfare check. But he's unlikely to engage willingly and likely to be angry / see it as confirmation 'the system' is against him etc so I wouldn't want to do that while still in a relationship with him.