Is there anything that I can to do to make him mine?
I've been talking to and sleeping with a guy who has a girlfriend is there anything that I can to do to make him mine?
The best thing to do is to date other guys and flirt. Be spontaneous, free and happy. Play the game of romance as a way of life. Flirt with that guy, outrageously. Be sexy, attractive. For him to want you, you have to give him a taste of what he could get. You can give him a taste by flirting with your eyes, seducing him.
If he feels this is only for him, he will reject it. He will think you are clingy, attached or desperate. For him to like you, you need to be ahead of him in one way or another. Develop you tantric skills. Guys want you when you have something they need. One of the key things guys need is pleasure, fun, enjoyment. If you can give him that, he'll be back running to be with you.
You see, it's all about being at your best and giving him what he needs the most.
This is playing fair and from an ethical point of view, by far the best option. Attacking your rival is not an option. Play fair and respect! Accept the outcome! Respect his choices and move on!