Your brother is a lucky man to have such a concerned younger brother. I can tell it pains you deeply to see him so lonely and upset but it’s also getting to you. You were right to ask for some help.
My best suggestion is that you talk to your parents. If you haven’t already done so, let them know how much your brother’s state is hurting you. You wrote a very thoughtful and articulate letter. If you can’t bring yourself to tell your folks, ask them to read your letter and this response. I think it would be helpful for you all to ask for a family consultation with your brother’s psychologist. His depression is affecting your whole family. You could all probably use some support as well as some guidance about how best to be helpful. As part of that meeting, you could discuss what treatment options (like medication) could be explored that might move things along. The psychologist will need your brother’s permission and, hopefully, his participation to make this happen. If your brother won’t consent to a family meeting, consider asking for a referral to a family therapist who can help the family learn ways to avoid getting caught up in his illness as well as ways to help him.
Another resource for you and your family is NAMI: The National Alliance on Mental Illness. There is a chapter in your city. NAMI offers support groups and resources (both in person and online) to families and friends of the mentally ill. It’s often a relief for people to talk with other families who are going through the same kinds of things. People share resources, give each other affirmation and suggestions, and are there for each other through the tough times and the successes.
I’m very glad you wrote to us. There is help available. You and your parents don’t have to figure this out all on your own.
I wish you well.