I have a shih tzu which has a dull, dry & has little fur. It doesn't seems like shih tzu which has double coating fur. I was thinking to shave all of his fur and let it grow back a new one.
Welcome to the wonderful world of poorly bred Shih Tzu pups. First, get your dog to the vet to see if she has any worms or any other health issues. Second, change his food to a high quality food like Wellness or Chicken Soup for the Dog Lover's Soul. Third, make sure that he doesn't have any fleas or ticks, and keep him on preventatives for them.
Many Shih Tzu have serious allergies that affect their coats. Once the allergies are recognized, food, fleas, etc. and treated, the coat usually comes back. Good luck to you!