Is he also a bit clumsy and poorly co-ordinated? In which case he may be dyspraxic. It is a problem on the learning curve of dyslexia, dyscalcula etc. DS and I both have it. Therapy varies. We had it about thirty years ago when the recognition of this problem was new and had specialised physiotherapyexercises, which helped - a bit ours and his salvation will be keyboards. See if you can get him some typing classes so that he can awe other children with his mastery of touch typing.
I know the problems, in the Vi Form, teachers were still constantly making me try different writing instruments and styles to try and improve my writing without avail. I hand wrote exams at O level, A level, and university and my poor writing never held me back.
So tell him, not to worry, lots of other people have the problem and got on in life and once he is 11 and goes to secondary school, he will never have to write another piece of school work. In the meanwhile, let him take his time and make sure the teacher isn't over preassurising him.