If you don't want to be outed then don't pursue any guys, gay or straight.

That being said, if you aren't out then maybe he just sees you as one of the boys and doesn't have any secret motive behind spending time with you.
Let him come to you, don't try anything in case he is legitimately straight; but if you decide to move forward with his advances you'll be outed eventually, even if he wants to be with you, that kind of stuff always gets out.