My brother is in the habit of thinking aloud when he is alone. He doesn't do this in public, only if he thinks he is alone. I often hear him when I slip in unnoticed into his room. Is this normal? Does he need medical help?
People learn in different ways. Some learn best by reading it for themselves. Others need to write it down for it to make sense. Some need to actually see it working. Others have to make it with their own hands. And some need to hear it. Your brother is in the latter group. Like all people, he can learn in multiple ways, but the way that works best for him is to hear something spoken. Thus, when he is alone and knows he won't be bothering others, he speaks his thoughts out loud so he can process his ideas better. There is nothing wrong with it; it is only culturally unacceptable and solely because it subjects others to hearing his half-formed thoughts.
I have often cautioned my brother about this habit, and he has shown interest in stopping, though he still does it. He says when he feels compelled two think aloud, he presses his lips togehter. Any sugestions on how to stop?
There is nothing wrong with what he does. That you don't like it doesn't mean he must change. Besides, you can't make other people act as you want them to act. You can only change yourself and encourage others in their choices.