Would he go for short bursts of as fast as possible, maybe? My DS (also 6, also has issues with writing) seemed to benefit from a few goes of the "ten minute concentration challenge". The rule was that I gave him a subject, and he had to start immediately and write as much about the subject as he could in 10 minutes, stop-watch timed. Didn't matter how neat it was or wasn't, provided it was legible, and didn't matter how interesting it was provided it was at least relevant to the subject - this exercise was unashamedly about quantity rather than quality. At the end of the 10 mins we counted the words (at his insistence, with mis-spelled words counting half, though I think I'd have ignored spelling if he hadn't insisted) and the challenge was to beat your personal best. He only did it a few times, but it seemed to help over a hump, by getting him unstuck, or something: he had been getting very stressed over exactly what to write. (Incidentally he didn't get beyond 20-something words in 10 minutes, even when I could see he was focused the whole time.)
