He should be seeing a vet without delay!!!
I've been through this when my lab was a puppy last year. He had a diarrhea caused by a cooked bone and it took us a while to figure it out. And he haven't had any bone ever since. We were at the vet last week for his shots and our vet suggested giving him raw bone to clean his teeth. I am still debating whether I should give him the Nylon type instead.