Turning over onto his back, he scooched down farther into his bag. It was the kind of sleeping bag with a hood built into it, so he pulled on the drawstring.
'Scooch' combines "scoot" which is to move, especially over, and scrunch, which indicates a crowding. So by using it, (and it is informal) the person is acknowledging that they are asking you to crowd yourself a bit more by scooting over to make room for them
scooch
Etymology
() Perhaps a portmanteau from scoot and inch (so as to describe a movement both rapid and short).