Just want to add that another point to note is that the problem often occurs when he has to write quickly (timed exercises) and that he hates writing. These two together, form a major obstacle to learning for many bright people- their writing cannot keep up with the flow of ideas in their brain.
So... how's about he's allowed to keyboard his assignments? Use a laptop in class? There is also fairly cheap software available now to automatically type what you speak into the computer. If assignments are assessing his thinking, then the school should have no problem with helping your son to find alternative ways to get his ideas down. (Also check out software like Inspiration - for 'brainstorming' ideas, and then organising them...)
I agree with previous comments, that lessening the pressure on your son is the major goal at the moment
