Hopefully it'll come out of his bottom. Check his gums and press them to make sure they go pale then pink again. I think that if they are pale and stay pale, it could mean that something is stuck. It's a good starting point before you phone the vet. I have seen whole supermarket bags coming out of dogs' bums before. It's not an ideal situation, but things do come out, and if you're worried, call the vet.