Hi there, I haven't really got much to add to what the others have said but I would try seeing whether he likes to use SymWriter. It is software that produces symbols above the written word to show the context. If you go www.Widgit.com and look on their products, you can get a free trail for around 30 days I think. Your son may be more interested in 'writing' if he doesn't have to physically handwrite something. The advantage of this software is that there he may be more motivated to 'have a go' as as the reward for producing a word is a symbol!
See what you think, obviously it won't solve everything but I work with a child who originally didn't want to write at all and will now 'write' using this software.