300 grams off a 1.3 kilo bun is a lot. You should start to syringe feed him. I dont know if you get Oxbow Critical Care in Brazil, or ay other recovery food fo small mammals (Emeraid is one brand). Those type foods come as powders that you mix with water and feed with a syringe.
You can soak pellets in water to a mush, but thts obviously a bit more work, mainly bc it can be difficult to get the paste smooth enough to easily pass through the tip of a syringe. If you have a coffee grinder Ive found that to work well for pulverizing pellets -just obv make sure you’ve cleaned out all remnants of coffee from the grinder first.
Dipyrone is metamizole. I had a bun who got that for stomach and dental pain, but it’s not a common drug here and it can only be given by vets -I got a small vial of it to take home bc it cant be bought in pharmacies even with a prescription. I cant find what the proper dosage is. My bun responded well to it, but there is apparently some controversy regarding its safety both in humans and animals. It seems dipyrone is very much used in Brazil though for both humans and animals, so your vets are probably very familiar with it. I don’t know if itd be worth discussing the possibility of changing pain meds to meloxicam, in case your bun is one of those few individuals that don’t tolerate dipyrone?
A rabbit should be meticulous about its personal hygiene, and when they arent, sth is wrong. Difficulty pulling his tail up sounds like sth to do with nerves. There shouldnt really be a risk of nicking a nerve during a normal castration, but weird things do sometimes happen. Can he walk/hop like normal?
If you can’t add pics/a video by clicking the Add Media button, look in your PMs, I will send you instructions. Pictures and videos are very helpful.