Well, it's pretty plain that he is genuinely depressed on a chronic basis. He sounds like he suffers from social anxiety that is crippling him from having normal connectedness to other people. That is very painful.

I think he is fortunate to have a brother who is concerned about him and cares that he may not be alright emotionally. If I had been you, I would not have put exactly what he wrote here on the forum. This is generally a safe and caring place and all that. But, I think it's best not to put out in public someone's private thoughts without them knowing and giving permission. I believe you meant well. Another approach might have been to say, "It sounds like my brother is worried about such and such." He sounds like a very sensitive person who could be hurt and upset easily. (I wouldn't tell him about posting his note, if I were you. -- which I don't think you intend, anyway.)

He may have deliberately left the note in the open, hoping that a family member would discover it and find out that he is in a lot of pain. I think you could start helping him by straight out telling him that you would be interested in hearing about how things are going for him. You could even tell him that you sense he is unhappy and that you are concerned. He may feel alone and not know how to initiate telling anyone how he feels.

I think the posters above may some good recommendations also. Good Luck.