Hi actually we literally just found it after hours of searching so he’s good thank you for your helpHi. I can't even begin to think that NOT taking him to the vet is an option. I mean, even if they can be somewhat digested by humans - in certain conditions - do you have any idea that the same is true for a cat?
If you think he can actually pass it, then just watch him and look for negative signs, like no stool, vomiting, lack of appetite. I am not sure I would want to wait for those things to happen, but you have a better knowledge of these stones than I do.