It might sound odd, but I would suggest explain your child the benefits of handwriting, one of which is improvement of academic performance https://ivypanda.com/blog/handwriting-good-for-your-studying/. Encourage him to try some exercises for improving his handwriting https://www.creativefabrica.com/the-artistry/craftshome/fun-exercises-to-improve-your-handwriting/
https://mycursive.com/how-to-improve-handwriting-for-teenagers/.
Your child may try mastering print handwriting which is tidy and neat and won't be hard to read for teachers. Plus it looks pretty cool, and could make his classmates jealous.