Practice calligraphy and sign up for hard-tipped calligraphy classes.
There can be some reward and punishment measures. For example, if you write well today, you will be rewarded with playing computer for a while. If you write poorly, you will not be allowed to play computer for two days. . .
Teach him to be patient and teach him to write slowly and not in a hurry.
Tell him how important it is to have neat handwriting. For example, if the handwriting is not good, the teacher will not be able to see clearly what he wrote after the test and will judge the correct question incorrectly.
In fact, how the handwriting is is secondary. The main thing is that handwriting can reveal a person's character. A calm person will write neatly, while a frizzy child will write messily. The most important thing is to teach your children to do things seriously and not to be impatient~