It could be the chloramphenicol. Some rabbits can have a bad reaction to a certain antibiotic, with it causing some stomach upset and nausea. You could ask your vet to try a different antibiotic if you think it could be this.
Penicillin g procaine/benzathine injections are often used for head abscesses successfully. But like I mentioned previously, I had really good success with oral azithromycin, though be aware that some rabbits do have severe negative reactions to it, but mine didn't. At most there was some initial stomach cramping for a few hours after administering, But after that they were fine and resumed eating. If his not eating isn't due to the antibiotic causing nausea, but is instead from mouth pain from the infection, azithromycin does have good tissue penetration and so acts more quickly than some antibiotics, but you do have to weigh it's use with the risk of possible stomach upset. But at this point it might be worth the risk for you. But if instead you want to try something more mild, usually the safest antibiotics with rabbits are baytril or smz/tmp, though just usually not as effective, or as quickly.
If it were me, I would at least want to try a different antibiotic before the final decision was made. But these are things only you can properly evaluate and decide with your vet. I'm so sorry you're faced with this decision. It's a difficult and gut wrenching one.
Penicillin g procaine/benzathine injections are often used for head abscesses successfully. But like I mentioned previously, I had really good success with oral azithromycin, though be aware that some rabbits do have severe negative reactions to it, but mine didn't. At most there was some initial stomach cramping for a few hours after administering, But after that they were fine and resumed eating. If his not eating isn't due to the antibiotic causing nausea, but is instead from mouth pain from the infection, azithromycin does have good tissue penetration and so acts more quickly than some antibiotics, but you do have to weigh it's use with the risk of possible stomach upset. But at this point it might be worth the risk for you. But if instead you want to try something more mild, usually the safest antibiotics with rabbits are baytril or smz/tmp, though just usually not as effective, or as quickly.
If it were me, I would at least want to try a different antibiotic before the final decision was made. But these are things only you can properly evaluate and decide with your vet. I'm so sorry you're faced with this decision. It's a difficult and gut wrenching one.