I've been in a similar situation with my best friend for about 8 years now.
We're the best of friends and share a lot with each but, at some point we started to 'harmlessly' and 'jokingly' flirt with each other. This started before he knew I was gay. However, after a while it goes from joking to actual flirting and I'm sure when/if we('ve) crossed that line.
At any rate - I wouldn't say I'm in love with him but, it does make things a little awkward when there's so much sexual tension. He knows I'm gay now and honestly, I've encouraged him to come out but, he sticks by him being straight. Who am I to not believe it?
For your particular situation, I wouldn't try and force anything. The longer you hold out for him, the worse it may get. Try and have a conversation with him about but, again, don't force the issue. You'll likely be better just leaving well enough alone before one of you gets hurt.
PS: There's another friend of mine who I'm in a similar situation as above, with. Now him, I'm actually attracted to. He's questing his sexuality, whether he's being truthful about that is another thing. He's saying he may be bisexual after he watched some gay porn with me out of 'curiosity'. Sigh - what a messed up group of friends I have.