Well, there's not much you can do in a situation like this. Hopefully the bit that he ate wasn't too jagged or sharp around the edges.
You can give him a few drops of olive oil to help make things pass more "smoothly". If you're really worried, give him a lot of soft foods. Baby food, apple sauce, un-spiced canned pumpkin, *cooked/softened* butternut squash, etc. Give him some soft foods with the drops of oil afterward and we'll see how we get from there.
Give him a warm
bath and massage his belly very lightly. I say do it very lightly because there's always the possibility the bit of rock could cut him, so just very lightly and gentle on the massages.
I know this is gross, but if he poops (there's always the chance of impaction) put on a rubber glove or something and squeeze the poop a little to see if the rock is in the poop. You want to be sure he passed the rock to ensure that everything is okay.
I hope all goes well! Please do keep us updated!