Does he have issues with the physical act of writing? does his hand hurt? have a weird pencil grip? OR does it seem more like a processing issue? have they tried to let him gather his ideas non-sequentially, i.e. through a bubble chart?
I would consider having him evaluated to figure out what is going on. He's already had plenty of opportunity to "practice" at school. IMO, telling him to practice more of what is not working will just make him more frustrated and may create self-esteem issues. It sounds like he needs to try something new. An evaluation may help you figure out the best approach.
