How do win over a boy that has a Girlfriend?
You don't. You leave them alone.
There are plenty of boys around, and if you have the time and energy required to trick a boy who's already in a relationship to fall for you, then you definitely can win a different boy, who may be worth your time more than one who claims to love someone else.
And if he cheats on his girlfriend with you, who's to say he won't do the same to you?