I think you should tell your boyfriend that the situation isn't fair to either of you, and he should tell his brother that he has a week or two (be specific) to find a job. There is no reason, barring disability, that he should be sponging off of you. I take it he isn't paying rent, since you said your boyfriend is supporting him?

Your boyfriend isn't responsible for his brother. You're all adults, right? If he tells his brother to find a job and a place within a certain amount of time, and his brother fails to do so, he should kick him out. I know it might seem cruel, but you can't let his brother keep manipulating you. Some people need a heavy dose of reality before they start taking responsibility for themselves. And such people will keep taking advantage of others as long as they're allowed.