Shih Tzu’s adult hair starts developing once the pet reaches a year. Meanwhile, you can give him good nutritious food plus omega 3,6 fatty acids supplements. Brush the hair regularly.
You can use supplements like coat star/ Nutricoat advance / Yafa goldcoat etc. Dry kibbles like Purina super coat dry food/ Purepet chicken puppy food/ Royal canin puppy food etc can be used. To discuss in detail, I suggest you to speak to kuddle nutritionist