Why were you snooping in his email?
Though this shouldn't need to be said, you should not have been reading his email! That said, now you know something that you shouldn't. How do you feel about his being bisexual? If it doesn't bother you, then there should be no problem and you should feel no need to let him know that you know. If it does bother you, then you should not continue seeing this person. His preferences will not change for you or anybody else. In any case, I don't advise that you tell him you did this. I know if I found out someone I was seeing was snooping around my stuff, they'd be history.
Not everyone that is bisexual is gay right?
I'm kind of scared cause I really love him and want to be with him. Being bisexual doesn't automatically make him gay right?