V.W.
Could be dyslexia... could be something along the lines of dysgraphia. Does he have pain when he writes? Does his writing difficulty cause him to expend an extraordinary amount of his brain solely getting the letters/numbers written? (If it takes a "typical" person 80% of their brain function composing what they are writing--the content or the math---and 20% for the physical process of writing the letters/number.... a dysgraphic would be using 80% on the physical aspect and only having 20% available for the actual content. I am making up the percentages, but do you see what I am saying?)
Also, those with dysgraphia tend to have awkward pencil grips and their spacing on the page is atrocious. They typically will be able to dictate a far more refined response than they can write. If his "only" writing issues seem to be general sloppiness and occasional reversals, I wouldn't assume dysgraphia at all. But there are other indicators that you may be unaware of to watch for.
Given that he has a diagnosis already, I would ask about screening him for dysgraphia as well. See what they say.