Rather than going into all the details, I would say, it does seem like an impossible job.
As far as UKBA is concerned, he has lived without his parents since 2005, and it proves he is able to live there. In about 3 years, you have only been to see him once, and it proves you do not play any part in his lookafter.
Having a lawyer is not a guarantee that you will get a favourbale outcome, but without a lawyer you have no chance and a refusal is guaranteed.
You need to see an honest solicitor who tells the truth. You do not go and ask a lawyer to make an application, because if you do, he will just make an application even if he knows it will be refused.
You go and see a lawyer and take his opinion if its possible to make an application. I am saying this to satisfy you. I know you will not succeed.
With what I know accurately about dependent relatives rules, you have no chance whatsoever of getting your brother here.
Being duff and dumb is not such a disability that he cannot look after himself if you send him money.
Once again, if you ask my honest opinion, then let me tell you, just forget about it. You haven't got a chance.
regards