Have there been any changes to yours and his lifestyle, any upsetting events in your lives lately? He may not be able to verbalize his internal upset about it. Kids don't always know how to verbalize fears and so he may be stuttering in response to some trauma. I don't really know but I do feel you shouldn't wait any longer to take him to see a professional. I am sure the stuttering itself is bothering him. A good speech therapist can help him and the sooner the better. Best to you and him.