I don't think that styrofoam absorbs fluid and he would have had to chew it to swalow it so i would watch his poops ..if anything seems not normal I would advise making an vet visit.
Yeah, you'd be surprised what my guys have gotten away with eating....
I agree, as long as he's acting normal and doesn't appear to be stopped up in the GI, no worries. The body can't really break down stryrofoam so it won't poison him most likely, and all it rally could do I think is slow his GI.