Is My partner Bi-ual?!?

I don't know what to do!! I think My partner is Bi-curious or Bi-ual. I found some e-mails regarding contact between each other when I was searching for something that he had asked me too. I confronted him about it and he claimed it was a friend that was joking around, but in the letters everything sounded just like how he types and speaks!! But this isn't the first time something like this has come up. We havent been dating for that long and everytime we have talked about it he promises that its not him and he wouldn't waste my time if he was. I REALLY love this guy and he says he loves me also. I don't want to keep harping on the subject with him but im really scared to keep placing my heart out there. HELP! what do I do.....