He sounds like he’s trying to hold on as logically as he can. Just stay calm, keep using the idea of making sure the “poison gas” doesn’t hurt him further. The biggest thing you can do is listen, don’t argue with him, and talk him towards logical nonviolent solutions. It’s hard if your parent’s aren’t on board. But you can only take on one family member at a time.
My biggest piece of advice is also take care of yourself as well. Find some support and ideas for you. You are also going to need a support net too. You love your brother, but he will hurt your feelings, he will do and say stuff he’ll be ashamed of later.
Don’t be afraid to get help in being his friend. My kid sister needed her therapist and support group when I was putting her through hell. It helped keep our friendship in tact.
May your holidays go well and may all be safe.