I am very sorry to learn of this situation.
I disagree with the statement you made "he does not qualify for special ed" - if he has a documented disability that is impairing his ability to complete his schoolwork (the technical phrase is "interferes with his educational attainment"), he will qualify for an IEP (Individualized Education Plan) that permits him to receive a "Free and Appropriate Education".
I would start by contacting the County Department of Education - apparently you are not getting very good help from the school itself (I have found that many times this is often just a result of having the wrong people at the wrong job - they may have many students in the school - some with severe handicaps - and they are unwilling to put in the time or effort to extend themselves simply because they feel overworked. It is a poor excuse, especially when it affects your child or grandchild, but it often happens, understanding that can help you better address the problem, especially when you understand you need their cooperation later - so when you "go over their head" I recommend doing so diplomatically if possible).
You can also get help from your local special needs community. Almost all counties in CA (or groups of counties) have networks of parents, usually associated with the Regional Centers (state offices supporting the needs of disabled children and adults), one of the things that these groups do is to help parents (and grandparents) approach the IEP process.
Your grandchild should be receiving assistance to address his needs in such a way that he can succeed in school - the accomodations he requires based on what you are posting are truly minimal. (Make sure they do not try to force "modifications" on him unless they are required - accommodations do not affect his ability to receive a H.S. diploma, while modifications do).