Hi there,
Well you must be pretty concerned! It is definitely not normal for a pup to not eat, especially for this long. I'm not sure exactly what your vet diagnosed but it sounds like they may have found that Billy Boy has an infection. Often the treatment for something like this may involve antibiotics and lots of TLC
The fact that you have found an ulcer on his tongue makes me think that this may be the reason he does not want to eat. He is just too sore! Ulcers develop for a variety of reasons, so another trip to the vet would be a good idea.
Let me know how you get on
Kate