Hopefully someone with more experience of the relevant dys-s can help, but my immediate thought is that it sounds more like simple asynchronous development and perfectionism. My DS6 also has the problem deciding what to write next (while being able to rattle away if he doesn't have to do the actual writing), and I think that's fairly typical of the age group. From school we're getting the message that he needs to work faster, but not the message that this is unusual! We've also seen the compulsion to rub letters out if they aren't perfect, though that's waning. I have sometimes just not let him rub a letter out, thought that takes judgement about when it's going to work... What I don't recognise from DS is the being unable to spell on paper what he can spell by saying the spelling: he seems to have no trouble saying the spelling to himself, if necessary several times, and then writing it down. What actually happens when your DS tries to write a word he knows how to spell? Does he then forget how to say the spelling, or does he spell it aloud but then refuse to write the first letter, or what? I'm having trouble imagining how someone can be able to spell a word aloud and be able to write letters, but not able to put the two together.
I'd be dubious about using a handwriting programme, personally, because I'd think that might just reinforce the perfectionism. I'd be keener on looking for circumstances in which he might find a little bit of writing useful for his own purposes - but now that I think about it, while I think this has been key for my DS, the ideas have all come from him, and maybe that's the main reason they've been useful! Would he go for writing you a message in code, for example?
