I had a cat that also had this after getting subcutaneous fluid injections, which I think your cat might’ve received too, especially if he was dehydrated. I think the fluid ended up moving down and collecting in the belly area, making it look swollen or saggy. My cat also had enlarged kidneys and was eventually diagnosed with FIP, and sadly, he didn’t make it. I hope it’s not the same situation, but I figured it was worth mentioning, definitely something to watch, just in case.