Does he want to improve his handwriting? If not, then no resource or program is going to help.
If the problem is that he forms the letters incorrectly (not just messily) you could try Handwriting Without Tears. The key to remediating handwriting is that you have to watch them like a hawk *every single time* they write anything and make sure that they *never* write the old way because you're retraining the muscles. For us (at age 9), this took about three months. If he is in school all day, this won't work.
If it is just a matter of writing too quickly or not caring, I don't know what to tell you. I have dealt with one of each--improper formation and not caring--and I was only able to remediate the former.
