If you know he is in the piece of rock, you might try making a small weak batch of water just big enough to cover the rock, and place the rock in there. He should run out looking for better water. Just a thought. Like hypo treatment.
Man IDK. If your sure he's in there I guess you could make up some water the right salinity and just wait him out. Just put an air pump on it to keep the water moving. I'm just guessing at this point. Its hard to get some creatures out. I have run my share of gametes.LOL Maybe in a day or two some food will entice him out so you can pull the rock out.