Frame challenge here: how about teaching him to type?
Both my son and I struggled with handwriting; we're both dyspraxic, although that wasn't understood when I was young. I'm also left handed, which made it even more difficult. My writing has always been scrappy and uneven. My son is grown up now, but he still can't do joined-up handwriting.
Learning to type made a huge difference for both of us. No more struggling with pens (leaky fountain pens were the bane of my childhood) or pencils that pushed through the paper, or hands that became sore from being forced into unnatural positions for hours on end. Also the pure mechanics of moving the pen or pencil take a lot of mental bandwidth away from the words that you want to write. Now the words just flowed from our heads through the keyboard and on to the screen. If you make a mistake, just backspace.
Before typing became an option my Mom used to take dictation from me, which I would then copy out. Later I got a dictation machine so I could separate the composition from the act of writing. Both of these things helped a lot.
Things have moved on a lot since then. Writing is still inevitable, but schools and exam boards are increasingly accepting of special needs such as dyspraxia. Large pieces of work such as projects can increasingly be typed rather than hand-written.
There are a number of typing courses available; we got our son on one at primary school (I think he was about 10), but there is no reason why you can't start earlier. A small portable keyboard may work better than a full-sized one.