How do I support a possibly closeted friend?

I’m gay and have been best friends with a straight guy for over 6 years. He honestly appears pretty bisexual, but has only ever talked about girls, and even little things like his behaviours and demeanour just make me struggle to believe he’s anything other than straight. He’s always loved and supported me, and we have a very intimate friendship. I won’t lie, his fruitiness has confused me at times, and I did develop feelings for him. But I got over them, until today. Another friend was talking to a gay guy she met, and as they exchanged socials, she saw he followed this straight friend of mine, to which the gay guy said he went on a date with- 2 years ago? He then had to leave, but that he had a bit to say about it. This has been pretty shocking news for me, as the guy I spent so long questioning but concluded on being straight has this lore. Part of me things it was a misunderstanding- and I’m waiting to see what this guy says next time he sees my friend. He’s just always acted pretty heterosexual, asking me silly questions about being gay, even joking about me being the gay friend to the duo. But this has totally changed my perspective on things, and idk what to do. I obviously would never ask my friend straight up about his sexuality, but it pains me thinking he has been dealing with this secret so long. Ik how it feels. Only he can confirm his sexuality, and I’ve told him I’m here for him, but I’m just so scared he’s living with this alone.