Not an answer I'm afraid, but just some advice - don't worry about this too much. It has to be readable or he'll have problems in exams, but past that, it really isn't that big a deal. Most people's handwriting improves over time. Messy handwriting is also a sign of increased intelligence! Yes, really - it can mean that the brain is moving so quickly, the hand can't keep up! (That's not technically accurate, but you get the point). A great deal of the most intelligent people in the world have writing you wouldn't believe.
And now I'll mind my own business...