How long does it take to obtain a double entry visa when staying in Shanghai for travel to Hong Kong

Peter, Australians with valid Australian passports can enter HK without visa. However, your son needs to apply for another X type if he wants to go back to study in Shanghai. Seriously, I am not sure if your son can get another one successfully when he is in HK because the authority requires that those who plan to apply in HK must provide their HK resident status. And he'd better ask his school if they are willing to issue another admission letter to him so that he can apply for another X type.