It sounds like it's either the food or that he's picked up a parasite. Pearl had runny liquidy stools and she had a parasite. I still don't know how she got it because she's an indoor only cat. I'd just bring in a stool sample and go from there. Good luck.

If he has IBD/Colitis he'd be having very bad diarrhea and he'd also be having accidents outside of the litter box. My RB cat Starr would just explode and he wouldn't be able to make it to the box in time. He also lost a lot of weight. He was very gasy though.