First of all, let me say how wonderful it is that you are advocating for your brother. It is so important to have advocates to help when you are dealing with diseases that limit your ability to move and/or to speak. You are providing an invaluable service to him and as one who has Parkinson's (and also a paralyzed vocal cord) I can certainly appreciate it.
You mention that his speech is weakening. Has he had any speech therapy? Speech therapists are among my favorite medical professionals as they offer great vocal exercises. Also, are you aware the singing can be beneficial for the voice even when speech is difficult? Here is an article about a singing group for people with neurological disorders, http://joyfulnoisechorus.org/
What type of Physical Therapy is offered to your brother, now?