I think it is great that you are looking out for your younger brother and trying to find ways to make him more comfortable. What I would recommend would be to let him talk and never try to finish his sentences if he is struggling to pronounce the words. I think the worst thing that a loved one can do is finish their sentences. I know the intent isn't too make them feel bad but it does not make the stutterer feel good when they fail to produce a sentence without someone aiding them. If you continue to tell your brother that his stutter has nothing to do with the person that he is then I think this will go a long way!