"You made a promise to Jehovah" ??? Oh my goodness how terrible for your brother. My son is high functioning autistic. He does not believe God exists. He held off telling my mother because he feared she wouldn't love him anymore. It's so messed up.
Your brother needs you. If you have the strength and capacity you must be his advocate and help him to freedom from the mind control. Make his medical team understand that not only is his mind wired differently which is hard enough in a neurotypical world but that he is trapped in a world/religion that will not recognize those differences and compound his problems and mental health.
I don't know your family but my heart breaks for him as I read your post.