I am sorry. Your brother is in pain and I can't begin to imagine how you must feel, being the only one that knows about his plan. How to go from there is a little tricky. I wish you weren't in such a position.
I attempted to commit suicide twice. One of them was a real attempt.
I have never once been not suicidal for even a day. Yet for the past 20 years I survived. And I never attempted again after I failed.
What essentially kept me from trying again was what happened when I woke up at the ICU unit.
When I came to I was filled with dread. A sort of doom that I can't even begin to describe. I was paralyzed. I could hear faint voices but I couldn't make out anything of them, they sounded otherworldly.
I had no idea what had happened. I didn't know who I was, where I was, why I was in so much pain. I was terrified to open my eyes and it was a real struggle when I tried.
When I finally did, I saw my mother crying her eyes out. She was laying on the hospital floor, pale and weak, almost in a fetal position and the nurses and her parents were telling her repeatedly that it will be okay, that I am strong, that I will survive, that I will open my eyes any second now.
As a side note that was the last time I saw my now-deceased grandfather.
I shut my eyes back immediately. It took me an hour or so to process what was going on. In all honesty I didn't want to wake up, I wanted to die even more now, seeing directly the consequences of my actions.
I had to wake up, though. It was unbearable to imagine what I would put these good people through if I just went and tried to off myself again.
Basically that is the only thing that kept me. I never stopped feeling suicidal, but I knew now that it couldn't be suicide. For years I rationalized that if I died it had to be because of something else, and so that way they wouldn't feel so guilty or devastated, hopefully.
Throughout the years I came up with several brilliant ideas that thankfully I never followed through. Things like staging a car accident or paying a hitman to do the job. Covid? Yeah I almost did that.
I do believe your brother can still be saved. Sometimes all it takes is seeing directly how someone cares for them, loves them, values them, and they would rather you stick around for another day.
It doesn't have to be the family.
Another idea is that you can forcefully intervene with his elaborate plan just so that he can't move on with it. Even if you have to steal those helium tanks so he can't use them, well so be it.
He could very well be just like me that will keep on having suicidal ideations, no matter what. But seeing how much pain it will cause others can be a reason why he would survive another day. He can find other reasons as well, but may need someone to show them to him.
It could just be that he realizes he may as well try to enjoy his time here. And really, when you come to the point where all you want is to die then you get enough courage to just do whatever the hell it is you want.
Point in case, after my failed attempt I quit my master's, my job, left my home country and moved to the other side of the world. Because what did I have to lose now.
Someone here mentioned microdosing mushrooms, and I second that.
I hope you may find something a little helpful in my ramblings.