Thank you for your help.
It could be that the handwriting problem is related to a learning disability, especially if the LD affects visual perceptual-motor performance (using the eyes and hands together). Children with what's called NonVerbal Learning Disabilities (NVLD or NLD) often have problems with handwriting, along with other problems. You should ask the school psychologist or the learning disabilities specialist in your district to evaluate your son's writing (and other visual perceptual skills) with this in mind. Also, it's important to let your son know that you know it's difficult for him to write, and to pay attention to his skills and positive traits. It should go without saying that there should be no comparison of his work with the writing of other children either at home or in school.