I agree with my colleagues that this is not a legal matter. However, I see that you are trying to help your brother and can offer you advice as a former school administrator. If you have questions about the grades your brother is getting, request a meeting with the administrator over his section / class and with the teachers about whom you are concerned. Ask to see samples of his work and a full accounting of why he is getting the grades he is getting. If he isn't doing the work, then he is earning low grades fair and square. If the teachers aren't doing their work, then it will be obvious to the administrator and to you. Ask them for recommendations about what your brother can do to improve his grades - extra work, re-take a test, get assignments in on time, get after school help from the teacher...the list can go on and on.
Your post doesn't say if you were able to get your brother's schedule changed or not, but if that is an option, it might help to give him a fresh start. Ask the administrator or a school counselor what other options you have. What does your brother say is the cause for his low grades? Your brother is lucky to have you in his corner - good luck!