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

You could have brought home a female bun and still be face with same question: what if it doesn’t work?… Males can bond. It *may* take a bit more time but I actually think if that’s the case with Gus and Oliver, that would be due to Gus’s age. He might be over 1yr old before you see him be more calm. Being a dutch bun, he may be a bit more energetic also, but some are chilled, it does depend on the individual rabbit.

That his presence has provided Oliver companionship – even when not together – is good. That alone gives you a chance to just take things really slow. As Mikey mentioned, wait a while after Gus is desexed. He’s still providing Oliver with a buddy, just being next door.

As to your concerns about getting attached and getting him neutered but then looking at rehoming if they don’t bond – I personally would look at it this way: Giving him up would be really, really hard. But, because you had him neutered, he would be in much better position to go to a new home as another rabbits bond mate. Not to mention, you would have prevented him breeding and there being more unwanted rabbits out there. I would consider the cost of the neuter like a donation to rabbit welfare.
If you had to rehome him, you could over see it and know the new owners are getting a more predictable rabbit then an undesexed one that they might then pass on to someone else, due to unwanted behaviours.
If it came to giving Gus up (hopefully it won’t ) and you then look to get Oliver another friend, look at getting an already fixed rabbit so you wont have to go through the stress of having another one desexed.

They very well may be quick friends after hes neutered and healed. Hormones can really make a bunny crazy, so when hormones are removed, they mellow out I think they sound like a promising bond once youre able to get everything started! Feel free to update us here along the way as well. Users here can offer support and alternate bonding tactics along the way to help you out.

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So Mikey and jerseygirl covered a lot of information (good job, everybody! ). The only thing that I would add from my own experience with bunny bonding is that, after pre-bonding, when I started putting my buns together, I started with super small bonding sessions of about 20 to 30 seconds each and then started working my way up to 1 minute, 1 minute 30 seconds, 2 minutes, etc… (I should mention, though, that I did a lot less pre-bonding than you’ll ultimately be doing since you’ll have to wait until your new bun is spayed, during which time your buns will have more time to get used to one another, so you may be able to get away with longer sessions.) A woman who worked in the animal shelter also suggested making sure that I gave enough space during sessions so that each bunny had a place to retreat to if they needed a break. 

I also looked for information during my second bonding experience, because I was having some trouble getting my big guy to behave, and one of the things that were suggested was doing some of the bonding sessions during the night, when bunnies might be more active. Some of my bunny bonding sessions took place between 11pm and 1am.

Lastly, I would add that each bonding experience is different. I’ve gone through two bunny bondings, and each one has had very unique challenges. With my first bunny bonding I didn’t have to do any sessions during the night, but I had to do them during the second bunny bonding experience. 

Oh, and be patient, that’s really key! 

Good luck!