O.k, if you just want to hide from him that you like him then behave to him as you would one of your other friends.(this also works to get him more comfortable around you)
If he's sad about his girlfriend flirting with other's than say your there for him and give him a hug or hold his hand.
This might sound a bit stupid but slowly and CAREFULLY hint to him that if she's flirting with other's, she probably doesn't give a damn about him. This is a very risky move to make, if you do this right, he'll break up with her and be free for you to move in. But don't rush into trying to go out with him after. On the other hand.... if you do this wrong he will probably never look you in the eye again, ever. He'll tell all his friends that your a b**ch, then they will tell their their's and so on until your next 2 years are ruined.ONLY use this as a LAST resort!
By the way, I'm a guy. Hope this help's!