im not the guy but I have been in your brother's position and now im in yours. The truth of the matter is that he is in a place where there's not a lot you can do to help him unless he wants to be helped, which depending on the person can take a long time. Which is obviously tough to deal with as an outsider because you will often feel bad when they lash out at you due to your attempts help but you have to realize he's not really mad at you he's mad at himself and his life. It really is a noble cause to take on because of that, I know partly you want to help him because it will make things easier on you when he's better but you could also have chosen to just ditch him with you haven't.

With that said the advice I can give you is to just keep offering the help and pick your battles of when to put your foot down. The one common similarity between the experiences of people who have fallen to the depths of their minds and climbed out is that it never happened for them overnight. Sit your brother down and explain the feelings that you wrote down here to him, he will most likely close up and get on the defensive, which is why you really need to come off from a place of understanding rather than accusatory when talking to him. Talk with him in his terms, when saying something start of with "I feel as if this isn't right" instead of "you're doing it wrong". Try not to use "you" period as that turns the focus on him which will make him put his guard up and what you want is for him to let you in. Once you feel like he has opened the door enough then bring up the possibilities for change. Recommend he goes to therapy, it really does help. However theres a good chance he'll tell you to fuck off with that so propose to him the idea of 'No Zero Days". This idea is essentially what has helped me and others who have gotten out of that situation. No Zero Days means essentially that, no days where you don't take a step forward into progressing. The key to this is making him choose something to start off with that is small, like making his bed every morning as soon as he gets up. Thats it. That'll be his one goal for the day, nothing scary or difficult about it. Then after a while of that add another small goal like now he also has to brush his teeth, shower and get dressed with clean clothes. Then you continue on with that until the goals get greater and greater. Idk your brother's situation and he may already be doing all of that, but the point is to start as small as possible because your goal is to go from no movement to just start to get the wheels turning again. What this does is that he will now have that sense of accomplishing something that he didn't have for so long. He will be proud of improving his life, which will in turn make him feel better about himself, which will in turn make him continue to improve. In times that he strays make sure to remind him of how far he's come, again not in an accusatory tone but in an understanding one.