As I say children that do this are usually upset, maybe its something you don't know about, what is he like at school does he do it there ?Threatening wont work, finding out why he does it will, get some help ...
i am his mum and its got nothing to do with him being upset,there has been no abuse etc in our home it is something he has done for a long time and it dosent matter what i threaten it is habitual
What does he mom say about him doing this , he is upset about something, take him back to see the doctor there will be a reason .