Welcome to my world! I bought a "Pekingese" from a backyard breeder (BAD IDEA!!!) and I got an oversized, 20 lb, snouted, gigantic, allergic dog with half the fur a Peke is supposed to have. Did I mention he has a severe underbite and bulgy eyes??? I keep kicking myself for buying him, and now I regret that decision like heck. My guess is your dog isn't purebred and like Bckrazy said, he probably only has a teeny bit of Shih Tzu(and, btw, Shih Tzus have snouts. They're just not as perceptible as a Lhasas, for example).
Muzzle - Square, short, unwrinkled,
That said and done, bad breeding of unregistered dogs has ruined a lot of dog breeds. I wouldn't be surprised if your "Shih Tzu" and my "Pekingese" are distant relatives