Sorry, Cassie, I don't think I've ever seen or heard of anything like this. It couldn't have been just "temporary", could it? Like maybe he was lying down on his tummy on top of a toy, etc and it made an impression in his skin?
If it wasn't that, then I have no idea. Was it red, irritated, etc? Sorry, just brainstorming and trying to come up with ideas. Hopefully, someone else will have better thoughts for you. [:)]