Hello! I am sorry to hear you have a sick puppy.
I would say that your situation is what commonly happens. As good as veterinarians are at diagnosing and treating conditions, sometimes we never truly find an answer to why something happened.
In this case, the exploratory surgery made sure there was nothing really bad going on - IMO, finding nothing wrong is just as important as finding something wrong.
The other thing to remember is that you still have biopsy results pending, and those tests may give you a true diagnosis, or cause of the vomitting and bloody diarrhea. It is also possible that they will all be normal as well, and not give a solid answer at all!
In my opinion, the most *awesome* news is that your puppy is doing so much better with symptomatic treatment at the vet clinic he is at! A lot of times, the biggest key is getting the vomitting and diarrhea to stop, so that it doesn't self-perpetuate.
So it is possible that there will not be a "specific diagnosis" by the time your puppy is discharged, but again, as long as the puppy is still improving at this time, which I would say he is since the vomitting has stopped and they are considering feeding him at this point, I would say he is in very good hands.
Good luck with your puppy!
Dr. Tanis!
Dog Veterinarian
10 years experience in small animal emergency medicine