The first place that came to mind was down the side of the car seat, the drivers side (which is why I asked if it was his car keys). So get him (or you) to go sit in his car in the driver seat & put your hand down the right side between the seat & door. I think maybe its either down in that little area either under the seat from falling out of his pocket or its possibly in the pocket of the actual door if the car has one.