Welcome, Ellie. And might I state what an exceptionally wonderful name you have?!
Your story sounds all too familiar to me, and your son sounds a lot like my brother. I'm not sure if he's still cutting, but if he is you do have reasoning for having him admitted to an inpatient facility (reasoning: he's a danger to himself).
It's a hard position to be in, I'm sure. But you do need to look out for yourself and the younger child, too. I know the more my family did for my brother, the more he 'hated' them. He hated me, too and I didn't even do anything.

I'm sure down the road, he will realize you were doing your best to help him, but in these situations - they never see it as help. So don't think it's you and don't take the grumpiness it brings (and it will) personally.
Hopefully you make yourself at home here and stick around so you can get some tips if needed and to let some of that stress out. I'm sure you're exhausted!
Again, welcome! I wish the best of luck for you and your family.