I have a shar pei + rottwiler pup who is 6 months old. He already is a good sized dog. My problem is that he w?

My problem is that he is shy around strangers. He will hide behind my wife or me when anyone he doesn't know is around. On occasion he has growled and made moves to bite people who insist on trying to pet him even after I have warned them. Is there any way to break or improve this behavior. He is a very smart dog and obeys commands well. I just don't want to get sued by someone who attempted to pet him after being warned. He is an adorable pup which doesn't help the situation.