Glad to see someone is looking out for this young man. I would first go to the school's ESE specialist. There may be some testing that needed in order to make sure he still qualifies for an IEP.
If all you need is a signature for a parent and can't get it, contact the school's social worker. She can go out to the home to get the required signature.
The school administration should also be able to help if needed.
Don't be afraid to make some noise. It sounds like you are one of a few who want to help this young man. It might seem like a minor point being that he is a senior. IEP accommodations can help him when taking SAT which could effect where he can go to college or end of course exams which could effect high school graduation.
If you have further questions, feel free to contact me directly or pose another questions. Good luck.