Plain Greek yogurt is fine for your cat. So is a probiotic like fortiflora. Canned pumpkin helps with diarrhea. Gerber turkey baby food may entice him to eat.
I may get him some to try. My mom's cat really liked the yogurt.
I have beef baby food and chicken and rice baby food. I offered him the beef and he licked it, but I guess he didn't care for the taste. He did eat some more dry food, though.
I thought I'd post an update about Tibby. He's doing MUCH better. Eating normally, sleeping normally, drinking normally. He still has runny stools, but he's not finished with his metronidazole. He's much more like his old self now, coming to my lap for cuddles and to watch bird videos on youtube. He's tolerating the kitten more, too. (Found them both asleep in my bed the other morning.) I've got my fingers crossed he'll continue to do well. He didn't want any greek yogurt, so I may try plain not-greek yogurt when I go to the store tomorrow.
A thank you to everyone who has commented and helped me out.