The reason may be in how brains develop: During the early school years, when kids are learning to shape letters, the nerve fibers that control fine motor skills in boys' brains typically haven't matured as much as girls' have. So the girls in your son's class may be better equipped to conquer penmanship.
Boys' brains eventually catch up and their handwriting gets better than their early attempts, but it's still usually not as neat as that of the girl at the next desk. That's because women tend to have more nerve connections between the two sides of the brain, which also helps with precision.
But reasons for the disparity don't end there: Social expectations also play a part. Nice penmanship is often considered a feminine trait, so boys aren't encouraged to improve theirs or care as much about how it looks.
What can you do to help? No need to make him practice. Just keep him supplied with a variety of activities, including those that boost fine motor skills, such as building Lego towers, cutting and pasting, and molding clay.