Basic really does sounds as if he is nauseated and I’m sure would benefit from anti nausea meds. Just tell the vet you would like to give him ondansetron as you are sure he is nauseated as he is usually such a good eater, and he want s to eat but can’t.
You could show the vet the SLGS dosing method and say this is what you would like to follow.
You can print a copy off from
HERE to show the vet. She might like to have a copy to keep.
Also just say you would like to try pain meds again to see if that helps Basic. Let us know how you get on.