My son is an extremely slow processor and also dysgraphic. He can spell orally but has more difficulty on paper. He has much more luck with using a whiteboard as he can erase and correct the letters he realizes aren't correct once he sees the word entirely written down. (Sort of his own multiple choice system, I guess; write it four different possible ways and pick the best one.) The chubby markers are also easier for him to write with.

What helped him immensely when he was 6 was jumping/hopping out his spelling words on a 'keyboard' stenciled onto a 3'x5' piece of canvas on the floor. His mind/body seemed to remember the jumping order and he learned the keyboard in no time, too. Using the keyboard for writing helps him out a lot, too. If your child is in school full or part time, ask for permission for keyboard use there, too.