Its either fold or push...he just committed half his stack. You have to figure that he's getting all his chips in this hand.

Therefore, you either fold, or get all your chips in and make him have a decision (instead of him making you have a decision)

I dunno about you, but I'm not strong enough to fold this...I'm not thinking he has AA or QQ, and if he has 55, I pay him off (you still have outs though)