The guy on the other end has no idea that your brother's speech is something he struggles with and affects him every day, he's just some immature video game dick who picked up on any weakness like a shark smells blood in the water. I can't tell you how many times people have said "T t t t today junior" to me without realizing they're mocking a pretty serious handicap. Remind yourself that words don't hurt and forgive him for his negative actions; he knows not what he does. Hanging on to anger is like holding hot coals waiting to throw them at someone, it is you who gets burned