I have heard that cats can get diarrhea from too much milk (ice cream would qualify!) I let Grasshopper have the bottom of the bowl lickings occasionally when I have ice cream or cereal with milk.. but very little. He should be OK, keep an eye on him. Poor baby!! (You, I mean, 'cause runny kitty poop is nasty to clean up!) Aww, poor baby to him too, what the heck!!
Hope he feels better soon!!