Your brother is lucky to have a sister like you; remember that!

The best you can probably do is to figure out what your limitations are regarding your ability to support him. It might include helping him study for his classes, making sure he is continuing to attend treatment while taking classes and possibly encouraging him to test the waters of school by taking a reduced courseload for a semester or two to see how he responds to the demands of school. His school should have some sort of support for him to compensate for his illness and he should see what's available. Ultimately, it will be best for him and the rest of the family if he is able to accomplish as much of this on his own as possible.