Your brother is fortunate, indeed, to have such a loving and concerned family. It must be very painful to all of you to see him fail to make a reasonable adult life. It is sad and frustrating, I’m sure. It may be that he has a mental illness or a personality disorder that makes it difficult for him to take responsibility for his decisions. Regardless, at this point he is so defensive that he can’t listen to even the kindest suggestions. The way he proves to you and to himself that he is independent is to be oppositional.
The only thing you haven’t done is respect his choices to live his life as he wishes. He does manage to get jobs. He hasn’t starved. He preserves his self-esteem by always seeing others as being to blame when things don’t work out. Although, from your perspective, he isn’t living an adult life, he thinks he is doing fine. As long as he isn’t looking to the rest of you to support him financially, it is his right to work or not, to eat when he wants, to leave any relationship that doesn’t suit him (even though it’s probably his fault that social and institutional relationships don’t work for him).
However, if he is expecting you all to support him financially, you do have the right to insist on his cooperation if you are to continue it. A clear statement about reasonable conditions for continued help is appropriate. But do make sure that you are willing to follow through if he doesn’t meet those expectations. Otherwise, such expectations are meaningless and won’t teach him a thing.
Therapy might help him but only if he engages with it. It is more likely to be helpful for you and your parents to seek out a family therapist to help all of you decide how to manage your brother. A therapist may be able to help you understand him better and may be able to give you some new tools for coping with his behavior and for helping him.
If you can’t find a family therapist where you live, consider joining one of the forums here at PsychCentral. People who have similar issues offer each other practical ideas and emotional support.
I wish you well.