Diarrhea can be caused by many different things but one of those is a change in diet. If the diet was not switched from old to new over a gradual time period, it can cause the stomach and intestines to be inflamed. This inflammation can cause diarrhea as well as blood in the diarrhea. This can be from eating the adult food but the pork bone is more of a likely cause. This is very strong for a dog to handle, especially if they are not used to eating this type of thing. Diarrhea is very common afterwards.
The same type of bloody diarrhea can occur from stress, such as moving to a new home. We call this stress colitis. In addition, we do not know if the puppy has been dewormed or vaccinated so that is a concern as well. Eventually he will need to go to the vet to have his fecal sample assessed for worms. In the meantime, we can start trying to settle his tummy.
This condition is managed with bland diet and medication. The bland diet can be cooked at home with bland, white chicken breast with NO SEASONING or lean cooked ground beef NO SEASONING and plain white rice. You can also use a prescription bland diet such and I/D, W/D or Royal canin gastrointestinal formula. I recommend small, frequent meals, to get the GI tract back on track. Do not continue to feed more food too quickly as this can cause a setback. Once the diarrhea is resolved, you can slowly mix in more of the dog food and mix out some of the rice mixture. This transition needs to be over a few days so we can watch for diarrhea to return.
Sometimes diet is enough to help but other times pets need a medication called metronidazole or metronidazole and amoxicillin to stop the bloody diarrhea. This needs to be prescribe from a veterinarian.
If further signs develop such as vomiting, decreased energy level or no appetite, further tests need to be completed to be sure something more severe is not going on, such as pancreatitis, something foreign stuck in the GI tract, etc.
Let me know when you return so we can discuss further questions you have.