Thanks for posting on HCM
Coupled to your brothers disability
he is suffering from a pyschosocial disorder that will require psychiatrist's attention. On my opinion his psychosocial disorder comes from two factors; your mother and his disability. Your mother seems to be the one encouraging this by trying to isolate/protect him and making him feel different, this causes him to actually realize that he has a problem and is different from other people he may meet. Given his age, integrating him directly to the society without prior psychotherapy
from a psychiatrist might result to more issues in public. My advise is convince both your mother and brother to meet a doctor. Be diplomatic and do know that your doctor's decision is best for your brother and society. If your brother get to spit or behave abnormally towards another individual with little patience, you might not be happy find him in a much more worst state. So getting him to a doctor is paramount.
Hope this helps