Brother can’t take care of himself. What should I do to help him?

If your county has an MHMR Board they sometimes have supportive housing apartments for those with mental health problems. They are income based.

Worst case scenario (where he's uncooperative, you aren't able to help and nothing changes): contact social services and report him as a vulnerable adult to get him on their radar. He might become a ward of the state but he will receive care and a place to live (probably in a facility). Also contact the landlord and let them know how he is degrading the apartment and this may accelerate the ward-ship process. I understand you are concerned for him and want something better, but he can only get what he allows for himself. This may be it.

This is a ticklish route to negotiate, because without your brother's permission they can't tell you anything, but if I were you I should contact the psychiatric hospital he was admitted to and volunteer yourself as a family contact. NOT as a caregiver, n.b.! but as a family member concerned about your loved one. Then at least if he's admitted again they can encourage him to share information with you, to help plan his discharge better.

Next: who is his landlord? Does he own the apartment, is there a building manager, is it part of any kind of community or housing scheme?

Are you able to visit his home for any length of time, or is that not doable because of the state it's in?

You are really up the creek without a paddle here. I am not certain what can be done. Everything is in the mix here from physical to mental illness, and unsafe living conditions as well as "being poor". There would have to be a guardianship and some sort of forced removal into "care" and with the problem of "poor and ill in the United States"? Well, I can only wish you the best of luck. I have a feeling that there will be very little intervention even with a call to Adult Protective Services, but they may be the place to START at least to gather information and to have them do a wellness check. Wishing you luck, and hoping you will share with this forum what you learn. We see so many problems and so seldom hear if anyone had any luck with a solution. Hope others will have more ideas for you.