i don't think there could be any "convincing" i think it would just have to happen... but if you get lucky enough to be in the same room as him and a chick, don't get too ahead of yourself and ruin it... he might be pissed at you later if you joined in (i'm talking like this is a drunken sort of thing... which seems to be the start of a lot of sexual openness.. at least in my experience)
I recommend you drop it. It sounds to me like he is more straight than you think and you should not push him into doing anything he does not want to do.
I don't think there is any point in trying to convince him otherwise. If he says he won't feel comfortable having you watch or join in, then he doesn't want you to. Trying to convince him is just going to piss him off, and you'll probably lose the friendship.
