I agree that you might want to find another vet, unless you are confidant that your current one is rabbit savvy. Did the vet discuss allergies at all? Sometimes sneezing is due to allergies, not to an infection. Has anything changed in his environment? Does he have new hay which might be dustier or is there a lot of pollen in the air, etc. Has he just been sneezing, or do you see any discharge? If there is a white discharge, that would probably indicate an infection. Sometimes a vet can culture the discharge and find an antibiotic specific to that infection.
That reaction he had was very strange, and it seems to indicate that he is allergic to the antibiotic or some component in it. It can take a while for infections to clear up, but you probably don’t want to keep giving a rabbit a medicine if he is having adverse reactions to it. Did the vet give him any sort of medicine with steroids in it? Maybe that would explain his behavior.