Get thee to a professional. I sounds as if the pup is protection aggressive. I would highly suggest you call a professional animal behaviorist/trainer to evaluate the dog and help you with a behavior modification program.

How old is your little brother? I would definitely not let him be alone with the dog at anytime.

In the meantime, I would put him on a Nothing in Life is Free (NILIF) program. The dog must work for everything. He should do a sit before you take him for a walk. He should sit before he gets his food. He should sit/down before he gets any petting. Have everyone in the house follow this. Do not let him up on furniture or your bed unless you or someone else in the household gives him permission (and then only if he has done a desired behavior first). You might want to put a lead or a head halter on him for better control while you are following this program.

Also, do not physically reprimand the pup - in a "status challenged" dog, that will backfire and would make him more nervous and aggressive.

Most dogs that show aggression do it because they are unsure of their status in the household and try to establish dominance over people they feel are on the same footing as them. Your pup needs to learn that the people of the house are on higher status.

I also have an aggressive dog (to my other dog rather than people thank God) and got a great deal of information and knowledge when an animal behaviorist came to my house to evaluate my furbabies.

Good Luck - and welcome to Pet Talk! If I can help at all, feel free to PM me.