I know nothing about autistic children, but I wonder if something like the "fiddle" quilts I made for Alzheimer's patients at our local nursing hone might be appropriate. I made a lap quilt out of fabrics of different textures and added pockets to put things in, a button that could be buttoned and unbuttened, some velcro, and a zipper to zip and unzip. I even added a string of spools (like beads). I didn't use actual beads because they could be swallowed if the cord broke. Just an idea.
the fidget blankets are good for autistic kids. But what I understand is that this young man needs something pocket sized that he can keep with him all the time. A quilt won't cut it in math class.