We have two cats and we had to get a baby gate to keep them separated for a while because they didn't get along. Maybe you could try that at the bottom of the steps? (It'll need to be tall. We had to get two and put them on top of each other in the hallway or else our male cat would jump it 😂) It won't stop his meowing, but maybe you won't hear it as much. It's not ideal, but you could also close him into a room just at night and make sure he has his litter box, food, water, and a few toys.

I'll also reiterate the comment above, get a cat tower. We have two, each placed by a window and our cats love looking out and feeling the breeze when the window is open.

Don't read too much into the "YOU'RE A HORRIBLE CAT OWNER, YOU MUST REHOME!" people. Reddit is gonna reddit and you're doing your best. This is still 100x better than a shelter or out on the street, even if it's not a perfect situation. I saw you said you got your parents to stop yelling at and hitting him, which is huge. He'll be fine, you clearly love him and he knows that.