My reaction to the letter was pretty much exactly in line with the answer. Maybe he's so depressed that his feelings were hurt in spite of everyone being truly empathetic and compassionate and actively doing their best to tiptoe around his feelings - maybe it's more of his depression coming out in feeling unwanted and dismissed. But even if that's true, IF she really has his best interest at heart and truly believes that everyone in the family is acting with the best of intentions, then rather than getting offended she should regard this as a potential miscommunication and try to learn more about how he's feeling and exactly what made him feel that way. Maybe he mostly needs to be listened to... maybe feeling closer to his brothers through some sincere communication is all he needs.

And hey, maybe the mom's right and maybe he's out of touch with reality and there's nothing they really could have done, but they're not going to find out or make anything constructive happen by telling him he needs to stop being oversensitive because his emotions are placing an unfair burden on his family and he needs to hide them: in other words, that his mother isn't a safe person and he can't rely on his family for emotional support and was being presumptuous to expect it in the first place.