Gus is aggressive because hes not neutered. Youre moving too fast for their bond to work, and skipping a few steps in the process. Youll need to go back a bit. Do not try to make a trio (yet). Trios are a lot harder, and being that youve skipped a lot, its unlikely to work too. I want to add one more thing, Ive had a trio of males and currently have a bonded pair of males who become depressed when the other is away. Sex of the bun doesnt matter, proper bonding and personalities matter
First step: Stop putting them together. The more they fight, the less likely they will bond in the future. Keep their cage set up like you have it, but do not allow them to interact aside from sniffing.
Step two: Neuter Gus. Hormones make rabbits aggressive. After he is neutered, he is going to need about two months to heal and drain his hormones.
Step three: Prebond, prebond, prebond! After Gus’s two month break for his neuter+healing, you can start prebonding. Prebonding is swapping their cages. Gus goes into Oliver’s cage and Oliver goes into Gus’s cage on even days, and on odd days they are back in their own cages. Youll let them mark one anothers things, which can be gross, but try to hold out cleaning as much as possible. Swapping them once a day like this allows them to get used to sharing territory and smelling “strangers” close to them. Youll want to do prebonding for atleast a month.
Step four: You can start bonding sessions in a neutral area once theyve stopped reacting to their cages being swapped. A neutral place to bond that many users use is their bath tub as most buns dont venture into them Youll start with quick sessions often and slowly progress to longer sessions less often (5 minute sessions every hour on day one; 10 minute sessions every hour on day two; ect). If they show signs of aggression, go back to a shorter time where they didnt show aggression.
Step five: Once they can go 5+ hours together in a neutral space without aggression, you can start bonding them in the area they will be sharing. Again, start with many quick sessions and slowly progress to longer sessions. If they constantly fight in this area, go back to step four and continue there for a few extra days.
Step six: Once they can handle their shared territory for 8+ hours, you should consider doing a 48 hour marathon. During this time, youll be with them including sleeping next to their shared cage. For aggression, use a three strike rule. If they show aggression more than three times, go back to step five for a few more days. If they dont show aggression at all during this step, after the 48 hours are over, you should be able to consider them bonded!
Signs of aggression: Biting, fur putting, mounting lasting for more than 10 to 20 seconds, lunging, boxing, growling, chasing.
Acceptable: Nipping, thumping, mounting lasting under 10 to 20 seconds