Hi Emily,
Instead of decreasing Prince's dose of Bup, what you can do quiet effectively is to give it to hi 3x a day, rather than 2x per day. That is how I did if for Jomo, always. It was a way to make sure that she was in as little of pain as possible.
Prince is feeling nauseas for a few reasons. Number one, he has ketones in his blood. You must keep giving him the sub-qs every single day until his appetite is fully back to normal. Number two, he likely has pancreatitis, which causes both pain and dehydration due to a lot of vomiting and sometimes diarrhea too. The Buprenor will ease his internal pain, and the sub-q's will also help TREMEDNOUSLY to revive his ailing pancreas. (Dr. Gardner, Jomo's old vet, told me this.). The sub-q's will also help with nausea related to dehydration.
The pills to help him feel like eating are good, but they do nothing to treat the underlying cause of his nausea. You must continue to give him the sub-qs, and if you don't see improvement at home with yourself giving the sub-qs, I would consider putting him in the hospital for at least 48 hours to really get those fluids circulating in him.
About his getting upset at the vet/hospital-- they WILL give him the Buprenor there, it makes cats very sleepy, and Prince will sleep while those IV fluids are being pumped through his veins 24/7 at the hospital, so his upset behavior will be gone. IV fluids is something that you cannot do for him at home.
You can try the Cernia for his nausea, but if it doesn't work right away, watch out. You've really got to watch him and make sure that he is eating everything. If Prince is not eating enough, or stops eating even with the cerenia, it's time to admit him into the hospital. He can slip into a full blown DKA case, as well as pancreatitis, plus fatty liver disease, ALL at the same time. This could happen not because of something you did, but because of something you couldn't do for Prince at home. If Prince won't eat even after you've given cerenia, it is highly possible that the sub-qs you are giving him are not enough fluids to knock out the toxins (ketones) from his blood, nor enough to relieve his stressed out pancreas.
You, yourself, must also be less fearful of taking Prince to the vet. He can definitely feel your stress.