If your bun is taking his meds as prescribed and on time, he shouldn’t be experiencing anything more than mild discomfort, although granted, that could make him grumpy.

I suspect that the real culprit, though, is the hormonal change his body is undergoing. He is, after all, suddenly without his gonads. For male buns, sperm in their bodies can live on for a month or so after neutering (yes, they can impregnate a female) and sometimes, as their bodies adjust, there’s a hormonal spike before everything calms down. This spike can increase territorial aggression, like boxing and scratching and nipping/biting when big hooman hands invade his space uninvited .

In terms of the eating: is your bun eating his fresh leaves and fresh veg/fruit, his pellets, his treats? If yes, and he’s just gone off his hay, keep experimenting with different hays, or use hay toppers like dried rose leaves etc to entice him, or even hide his favourite fresh treats in a bit of hay to get him going. Hay is vital and needs to remain a bun’s main source of food.

If he’s off all food in general, have the vet see him asap. Buns need to resume eating and pooping within hours post neuter.

((((((good luck))))))

It is, as Q8 suggests, most likely he is going through a post neuter craze, though pain is possible too. Keep an eye out for any noticeable swelling near the incision site. Although post neuter abscesses are rare, they can happen. This is unlikely, but given how aggressive he is, it would be worth checking him each day til he calms down, when you check how everything is healing down there, if you can manage it. Post neuter abscesses are more common in guinea pigs than rabbits, and I’ve had first hand experience of one in a piggy, which is why I’ve mentioned it as a possibility. Hopefully he is fine, apart from his hormones, and will start to calm down soon!