2 Answers

The Shih Tzu is an alert, lively, little dog. It is happy and hardy, and packed with character. The gentle, loyal Shih Tzu makes friends easily and responds well to consistent, patient training. It makes a very alert watchdog. It is courageous and clever. Playful and spunky, this affectionate little dog likes to be with people and is generally good with other pets. Some can be difficult to housebreak. The Shih Tzu needs all of the humans in the house to be pack leaders, with the rules of the house made consistently clear. Owners who allow their dogs to take over may find them to be snappish if they are surprised or peeved. Because of this dog’s small size and its adorable face, it commonly develops Small Dog Syndrome, human induced behaviors where the dog believes he is the boss of humans. This causes a varying degree of behavioral issues, such as, but not limited to separation anxiety, guarding, growling, snapping, and even biting. These dogs may become untrustworthy with children and sometimes adults, as they try and tell the humans what THEY want THEM to do. They will be obstinate as they take their stand and defend their top position in the pack. They may bark obsessively as they try and TELL you what they want. These behaviors are NOT Shih Tzu traits, but rather behaviors brought on by the way they are treated by people around them. Give this dog rules and limits as to what it is and is not allowed to do. Be its firm, stable, consistent pack leader. Take it for daily pack walks to burn mental and physical energy. Its temperament will improve for the better, and you will bring out the sweet, trustworthy dog in it.

More  >http://www.dogbreedinfo.com/shihtzu.htm

Have you had him checked by a vet for a possible medical condition that may be causing him to have a temper?