Have you checked to make sure that he does not have a sore spot on his head, if this is a new thing, it may be that he is afraid of being hurt by someone. Another thing is that a lot of small dogs are scared of something coming from above, so have the people first put their hands under his chin and scratch him and then go up to his head from the frot of him, rather than from behind. I don't know if either of these will help, but if he never did this before it sounds like something happened to scare him.
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I have a 1yr old male yorkie who has just started to bite people if they pet him on the head.He has just started doing this.He doesn't bite anyone he knows just strangers.He will let them pet him anywhere except the head.If they are petting him and move their hand towards his head he bites them.How do I break him from this?He goes with me a lot and I don't want to have to start leaving him at home.Please help.