clearly and firmly tell him what's what. You aren't going to be a mistress for him. He may also like you but while he has a girlfriend he is off limits.
Tell him that you're not comfortable with the certain act that makes you uncomfortable and tell him that you don't want to be the other woman so out of respect for the girlfriend he needs to respect those boundaries.
If this is more about you finding out if he likes you, you could just pose the question, "hey you're always teasing me and trying to sleep with me, are you just joking around or is there more to it" ... or something along those lines
You obviously can't be his friend, neither he can. There is attraction and you cannot do anything about it. But there's a reason why he's got a Girlfriend other than you, and for your "friend" his Girlfriend might be more special for him than you. Bear in mind that just because a guy wants to get into your pants doesn't mean he loves you.
How to handle this? Not much to do. You can't be his friend nor his GF. Perhaps getting a new best friend? Preferably a female best friend.
In my opinion, if he is always trying to sleep with you then that's not a definition of a best friend. If you like him I think you should just tell him instead of trying to hide it because I don't think you can be his friend if you love or like him so much on that level.