If he is still eating and drinking, and acting normally I wouldn't worry. I have a dog who used to eat q-tips, whole and they would come out in her stool like that. Not fun but she was never harmed by it. Popcorn and cranberries are both non-toxic to dogs and assuming the string was cotton or similar, there should be no issue with that either. Dogs tend to chew up random things as puppies (Underwear, trash, ect.) and swallow some of it all. So chances are, he's eating cotton before. As long as he didn't choke eating it, there should be no problem.