For example, a student of mine who had done little work all year was very motivated when the coach and I would not back down to let him play baseball due to his failing grades. He sat on the bench until he had established a passing grade, despite the pleas of his mother.
There may be another adult with which the student has a good relationship. Positive reinforcement can come from anyone. Use this to your advantage by requesting assistance from other teachers, aides, librarians or even a secretary.
Students need to know that he has a support group, but also that everyone is on the same page. The more consistency in the student’s life, the better the results.