I think this should be entitled “Problems with grandson’s self confidence”
Poor little boy.
For any child to call himself “stupid” is the saddest thing. He clearly has many talents- all children have talents- and he needs to be praised for what he is good at. To over stress what some see as a shortcoming at this stage in his development is not going to erase the feeling that he is “stupid” but reinforce it.
There are books to help with handwriting skills, I have used them with teenagers, but his teachers should be aware and doing anything necessary, perhaps he has not been properly taught. Encourage drawing and colouring, encourage word puzzles, mazes, word searches, and anything to do with language which does not depend on his handwriting.
BTW I can think of many teenagers (usually girls) who could do “pretty, pretty” handwriting, but the content was way below that of others whose writing was less “attractive”
However, the bottom line for me would be to excise that word “stupid” from his vocabulary.