I agree with Sandman2. If the doctors ruled out medical problems, then this behavior could be the direct result of an emotional issue than began approximately 6 months ago. I would encourage you to have your child evaluated by a mental health professional to rule out emotional issues that could be triggering this behavior. The school counselor, psychologist, or social worker may also be able to provide this service at no cost to you. Instead of getting mad at your son or disciplining him for this behavior, it is best that you rule out the mental health component.
Well, there may not be anything physically wrong with him, but something is certainly going on. Anxiety, among other things can do this. Have there been any big changes in his/your lifestyle in the last 6 months?