The fear of rejection & ridicule is strong in most of us, especially from valued friends. These fears are reinforced intentionally by homphobic remarks, behaviour, boasting etc., & unintentionally by thoughtless remarks, jokes & the likes. As gay people we live with this all our lives & it is often not obvious to even the most sympathetic of straight or bi-guys.
I came out nearly forty years ago to a group of straight life counsellors & was in a cold sweat, even though I had the support of a couple of gay friends at the lecture. In my community I am well known & accepted, I don't conceal my sexuality but don't flaunt it either & yet, I still have twinges of worry if asked in straight company " why have you never married ?", when I don't know what the reaction will be.
So you see it is important to understand how hard it could be for your friend, if he is gay, to take that giant step & actually say it. It is imperritive not to put pressure on but rather ease his fears with subtle, & I emphasise subtle, signs of acceptance of gay people. Commenting favourably on a film such as Broke Back Moutain, inviting a gay aquaintance to your group occasionally, & condemning homophobic behavior, are just a few of the ways you can ease his fears . But do let him proceed at his own pace, even if you confirm your suspicions, let him keep up his charade, his security blanket.
Beware of revealing a personal gay experience or, of concocting a story, because this could, as has been suggested by another writer, be misinterpreted as a come on.
I commend your concern for your friend & hope that what ever you do works out.
Regards to all,
Richard.