Is he really bisexual?

Ok, so I have this friend I've known since high school that is a real looker and popular too. I've seen his bare butt before and it's a lovely huge ass. I do have sexual feelings towards him, obviously, and here is what I want to know:

First off, he is into girls and has dated in the past, including marrying the mother of his 2 sons. He had taken shirtless selfies in the past and both sexes have "liked" the pics, including the ones he took recently. The girls on there will comment with the obvious sexual comments and the guys will too, and my friend will "like" their comments, even the guys who say things like "sweet nipples" and "now I wanna fuck you". Is my friend just comfortable with his sexuality or does he really "get off" on both sexes making comments like that towards him?

I have also noticed that he is currently dating but hasn't listed who his partner is and never talks about them or posts pics of them together like he has with other girls in the past. I don't even know if it's a girl he's dating because it's not that obvious. I wonder if it's a guy because he keeps it so secret and doesn't want his whole circle of friends to know.

I hesitate on asking him about his sexuality and/or his dating life because I don't want him to feel uncomfortable, get offended or remove me, but I just have this urge to flirt with him, at least playfully and not full-on hardcore flirtation, and would like to know so I could possibly talk to him about men.

If you must ask; yes, I love flirting with male friends (at least those who are ok with homosexuality/bisexuality).