Another idea is to allow him to do the school worksheets while watching TV, or listening to a book on tape or favorite music. Not sure that I would want to start down that path, but if he is willing to make an effort to stay within the lines, you've got a built in reinforcer.
It sounds like you are starting to accept that he isn't likely to get much satisfaction from K level assignments, and that's very important.
I also like graph paper...perhaps he would like to draw a B cell on graph paper and you could show him how to turn that into a knitting or crochet pattern.
Smiles,
G
