I've heard many things about cats not tolerating milk (yes, ice cream would qualify). I think it's only if they don't get it on a regular basis though. Tubby used to get milk practically everyday when "grandma" was taking care of him. Then when I moved in with Terry, it became a weekend thing where every Saturday and Sunday Tubby would get the left over milk from Terry's cheerios. Tubby never got diarrhea at all. The only ill effects ever seen were when Tubby thought we forgot about it being Saturday and milk time for him. He would be sure to remind us in no uncertain terms. We've had to stop this weekend ritual though because of Tubby's failing kidneys. Now even if he gets two licks of the spoon he throws up within 10 minutes, doesn't even get near to being diarrhea. Sooo, needless to say he doesn't get any at all anymore - none - zip - nada - noway! I don't think there would be any permanent damage done. It's just as if you had eaten something that didn't sit right. As soon as it's made its way through your system, you're over it. But I would keep an eye on him anyway, just to make sure. He's so young that you don't want to take any chances.