Well his regular vet finally called me back. She concurred that there's probably something awry with their scale or how he was weighed - perhaps his paw was off or something. I know I can tell that he weighs more than Mitzi and she's a tubbette at 11 lbs.

The vet said she would recommend ultra-sounding him again before considering surgery to see if the stone had grown at all from the last time they spotted it on ultrasound. From what they saw last time, the stone was way too small to consider surgery as they didn't want to open him up and have to poke around for something that they may not find. As all his blood and urine tests came back normal they have nothing to go on as to what this stone is made up of - there are no crystals or anything like that to give a clue as to what we're dealing with. I can't afford an ultrasound right now; I will check with IMOM to see if they might be able to help out.

I will continue with the Amoxycillin and pain meds as needed. He will be rechecked in about three weeks.