You don't... just find someone else.
Now I cannot tell you what to do. What you should do is what you feel you should. But, I personally think that you should tell him how you feel. Don't under any circumsatnces expect that he will drop this other girl for you. Because if he doesn't pick you it can be painful for you to deal with. But if he does pick you, it will make your decision sweeter. This is as long as he has a Girlfriend and is not Married, if hes married leave well enough alone. But if not hey, alls fair in love in war!! Good luck hope everything works out with or without him :)