M.,
I know you already received many responses, but I wanted to let you know what helped with my son. I am a homeschool mom and our son struggled with writing, still does sometimes, but we worked on writing with his fingers until his hands were ready to hold a pencil. Some of the ways you can do this is to put shaving or whipped cream in a pan and ask him to draw lines, circles, and other shapes for you in the cream. Show him how to make the shapes for his own name, then for your name, and then his sister's name.
You can also use different mediums like dried rice, wet sand, etc. to draw with his fingers. To strengthen his fingers and palms you can offer play dough and ask him to create different 3-D shapes such as a cube, sphere, cone, etc. Draw with a stick in the sand or use large sidewalk chalk and make different drawings.
Work on cutting skills and coloring with crayons, teaching him different ways to hold his fingers. He will not be very good at these techniques at first, but let him try, and even if he is not cutting or coloring perfectly don't step in and do it for him or it will silently tell him he's not good enough to accomplish the task on his own.
He most likely won't have a very long attention span for many of these activities, expect maybe the play dough. So don't force him to sit longer than he can. Once you are done with practicing a few shapes or letters, then let him play on his own. Over time he will start practicing the skills you have been teaching and slowly gaining mastery.
Good luck!
C. J.