You can’t be reinfected with Mononucleosis (mono) because your body has created immunity against the condition for life. I don’t believe that your boyfriend can be infected with mono either because it seems that he has been already exposed. He might have had mono in the past and also developed immunity or overcome the infection without symptoms. Mono is a fairly benign infectious condition, and there's nothing really to worry about, although your consideration for his health is admirable.