Echo the advice about contacting GP and/or police, and preparing to split up. Your own safety and wellbeing comes first. Even as a hobby which has spiralled out of control via dodgy internet sources, gravitating toward weapons would bother me.
Beyond a few weeks supplies, which is sensible for everyone, being prepared for the uenexpected is less about stuff, more about practical skills and psychological resilience. So you could try being very dry and matter-of-fact with him, see if it redirects his focus. Only if you want to, this not your job.
So. Is he a generally competent person around the house? Does he keep his family and home clean, fed and organised, while he still has power and water and safe streets? If not, he should probably practice this first. Can he carry out basic household repairs: not just leaking tap, change tyre, fix shelf; but also sew buttons, change a broken zip, darn jumpers? How are his first aid skills? Not a fancy kit, but thorough, recent, accredited training in what to do in all common eventualities. Is he as healthy as he could be, eats sensibly, fit and active? Somewhere online there's an account of a group of preppers in America doing a dry run of societal collapse at someone's ranch - they spent crucial survival days painfully detoxing from their various addictions - alcohol, cigarettes, caffeine, sugar. Is he on any medications which would be reduced or eliminated if he ate and moved differently?
Just for starters. Lots and lots to tackle, most of it helpful to family and society in the meantime, and none of it involves massive stockpiles or weaponry.
Sobering accounts from those who lived through civil war in Eastern Europe might be another wake up call. Pulling together, forging communities was important; lone wolves didn't fare well.