S.M.
Instead of having the paper flat on the table in front of him, try getting a big roll of paper, and putting some on a wall in front of him, or on a sturdy easel. Then he can use markers to make BIG letters, and the different angle for his hand may feel better. When he conquers that, move on to a slanted surface. I assume he has an IEP at school, so they should be willing to make these adaptations for him there as well.
To build the muscles in his hands, lots of playdoh and clay work is good. You can work up from the softer playdoh to some of the harder clays. Have you seen those hairy rubber balls they sell at walmart and other places? When you squeeze them, they "bulge" out, and kids think it's really cool, and that encourages squeezing their hands and can help build muscles in their hands.
Working with scissors can help. Cutting paper, or building playdoh snakes and then cutting them.
Operation! This game is all about the pincer grip. You may want to let him play without turning the buzzer on at first. The sound may scare him enough he won't want to play again.
I wonder if his hands hurt to do these things? Could the muscles be tight in his hands from the SB? What about getting one of those bed buddy things warmed up, and letting him squeeze that for a few minutes before trying to write? That always feels good on my hands when they're stiff or sore. (A bed buddy is just a tube sock filled with plain white rice, and you nuke it for a couple minutes and it makes a nice heat pack)
My friend's son with SB has trouble writing as well. She pulled him out after about 4th grade to homeschool him. She said it is fairly common with SB, and I believe she called it dysgraphia. Dysgraphia is to writing what dyslexia is to reading. A google search will bring up lots of websites with ideas to help.
My friend lives in Holland, Michigan, and I'm sure she'd be happy to talk with you through e-mail about your shared experiences. Her son Justin will be 14 in June. Let me know if you're interested.