Depending on your son's age the Alpha Smart Neo (despite being discontinued) might make sense as an extremely bare bones word processor. I used an earlier model

Depending on your son's age the Alpha Smart Neo (despite being discontinued) might make sense as an extremely bare bones word processor. I used an earlier model when I was in grade school and it was convenient for busting out first drafts and some light editing. The trick of the device, at least back then, was that it would present as a keyboard to transfer the text to a computer.