Assuming he has a diagnosis, the most important thing he needs right now is to apply for disability benefits, and get in touch with your local department of vocational rehabilitation (assuming you're in the US). They will support him in finding and keeping a job. Disability benefits can take upwards of two years to approve, but the wait may be shorter for him because autism is directly listed under "qualifying disabilities". I would also call child protective services and let them know what's going on, because they can help him find housing either by legally forcing the grandparents to keep him, or by taking him to foster care until he's 18.
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"Survival is insufficient" - Seven of Nine
Diagnosed with ASD level 1 on the 10th of April, 2014
Rediagnosed with ASD level 2 on the 4th of May, 2019
Thanks to Olympiadis for my fantastic avatar!