It's also not too early to teach him how to type. Diana Hanbury King has a great typing program, also available from Educators Publishing Service. Once he knows where the keys are, you can get him a typing software program to practice what he's learned and build his speed. Type to Learn is a popular one that kids enjoy. I'd probably work on both his handwriting and his typing skills to attack the problem from both ends.
Handwriting Problems, ADHD, and Self-Esteem
LD and ADD/ADHD Expert Advice from Eileen S. Marzola, Ed.D.