I'm not going to try to diagnose Will's learning ability or dis-ability from one picture on the Internet. But in the photo, he had written not on a flat piece of paper, but on a paper on the wall. It's not easy for an adult to write legibly, let alone a child. Note that he spelled every word correctly, except for one - "doges." And he remembered to dot his i's. I've seen much, much worse in 3rd graders.
I'd be interested to see if he has trouble with math, or if it's just language arts. Some kids are better in one, and some in others.
I have always been impressed with his curiosity, his interest in how things work, and his endless questions, "why?" I think that's a far more important measure of his abilities than his handwriting and spelling.