Grid inside face in After effects CS6

I am looking for some help with an effect that I am trying to create in after effects.

I filmed about a 3 mintue video of guy rapping in front of a grreen screen. The shot is really just his face and he is moving it as he raps. I removed the greenscreen background in After Effects.

What I would like to do now is place a grid that only shows up on his face. So that it looks like the grid in inside or on top of his face. How can i do this other than masking the grid frame by frame which will take way too long?
 

Apreciate any help!