You mention “writing assignments.” Does that mean “handwritten” or “reports, essays, etc.?” Is your son experiencing writers’ block? You might want to search that topic in the ADDitude articles.
For studying, there’s an article about “make a practice test” that might help. Or someone could try discussing the information with him — the main ideas of each paragraph in a chapter, for example. The ADHD mind likes interesting and engaging activities. Sitting and reciting lists can be quite boring.