I am not sure but I believe they can go for awhile without eating (unlike cats). Your dogs symptoms sound a lot like pancreatitis, though, which can be deadly (which I unfortunately know about first hand). Your dog needs to see a vet. I would not force-feed the dog. When my dog had pancreatitis she could not eat or drink and had to have IV fluids. I read up on it and I do recall that dogs with this probably should not be fed, their digestive system needs a 'break' and they can go without eating for days, sorry I do not remember exactly how many. I'm not saying this is what your dog has, but really he needs to be seen by a vet and asap. Hope your dog feels better soon!