I relate to what you have written. You did not say how old or what grade your child was in so my comments might not apply.
Next time the teachers complains they cannot read his writting you might ask them for more information. Or you might want to bring it up to them before they mention it to you again. Play out in your mind and find your words before you confront them. I might say something like "I have been thinking about your comment that you not able to read my child's writing." "You know you are not the only teacher that has asked me that." Then say. "I would like for you to tell me why noone can read his writing." "I would like to request some testing be done by the school to determine if he has a learning disability."
Just a note to let you know that many children cannot write or spell and they will still be very successful in life. However, it is the schools responsilibity to find out what they can do to help this child accomplish his writing skills and if they can't then by law they need to test him.
My heart goes out to you as I have experienced some of the same issues. Just give your child encouragment and help him with his writing if you can. Parent usually are not able to be very much help to a child that is not interested. Very few parents can tutor their own children successfully. Especially if the child does not seem interested.