HiThis is a frustratiing situation.
First, you can ask for whatever you want!! I am a big believer in asking for what you want. Having said that, a bit of investigating may be in order. There are many laws that govern what schools can and cannot do. Some laws are federal, some state and then sometimes school districts are left to make decisions themselves. Giving the school the benefit of the doubt, I would assume that your sons placement was an oversight. Perhaps this issue slipped through the cracks. The first thing to do, is call his counselor/advisor and find out who made the decision for the placement and why they made that decision. It is possible that the school did not feel that he was ready to move up given his performance, but that needs to be clarified. As he was out on medical leave, the school should have made arrangements for your son to complete his work. Was there a liaison between you and the school during his leave? Did he miss other assignments or tests?