If you are concerned, there is probably something to be concerned about. Keep after it until you are satisfied and comfortable that what you have a good explanation for what you see. Is it just writing or other there other unusual language behaviors? How does he do with pictures, reading? Is his vision OK? How is his hand/arm strength for other activities such as peeling fruit, dressing, etc? Think about anything else you see and try to get a full picture of what's going on. Whether or not there's a disability, it will be helpful to know.
http://www.readingrockets.org/looking_at_writing/Listed above is a website with real writing samples and guidance on teaching writing.
