Don't keep on at him, if you are not careful he will soon become turned off entirely and not want to do the work at all! Try to remember in some countries children do not start school until they are 7, by which time they have all the skills set in place to take off academically. I would praise him for the work he does and the fact that he is clever, but he might find writing very tiring and it may make his arm ache. Try giving him a pencil grip and changing the height of the table he works at. Make sure his feet are on the floor (put books under his feet if necessary. If he holds the pencil very tightly and presses on hard this is most certainly the problem with his writing. Hope this helps good luck.