Sounds like it is a good preschool, how is your son now? Is he still refusing to go to school?
Other than writing, does he like to draw or coloring too? Does he like to make up a story? May be try to encourage him to do the worksheets, e.g. tell him that he can write his own bk if he knows how to write the letters, it is fun. If you like the teacher and like this preschool, seems better to keep him there. Do you know is it a lot of worksheets or just a page? I think a page is fine for them to start learning letters.
