Is it permissible to enroll my deaf brother in a school where the majority of students are Christians, after he has completed the sixth grade, for him to continue his studies there?

The young one is greatly influenced by his teacher, and a Christian teacher will instill Christian values in the child, leading the child to imitate his teacher in many things. The corruption of losing religion and values is not equal to the corruption of leaving a child without school education. So, do not send your child unless you know that those teaching him are Muslims and that the curriculum does not contradict Islam. If you do not find that, then his staying at home with an untainted innate disposition is better for him and for his parents.