My recommendation would be not to do it. There is extremely high risk involved once the baby bun hits puberty, of a serious fight breaking out. Intact hormonal male rabbits will almost always end up in a fight.
These aren't the gentler hierarchy fights to just establish who's in charge and who's subordinate, where injuries are unlikely to minimal. These are the all out, territorial dominance, 'I'm going to try and kill you' type fights, where they try and latch on with teeth to rip flesh, and get to the belly with claws to eviscerate or neuter their foe. Make no mistake, these cute fluffy animals can cause extremely serious injuries when they want to, even to the point of killing.
So while there may be a very slight chance that two intact hormonal males could possibly get along without wanting to kill one another, it's much more likely that the older male may start trying to excessively hump the baby as they get used to each other, and end up scaring him off. Or if your older bun does stay calm and doesn't harass the baby, then the other likely scenario is in a few weeks time, that baby bun is going to hit puberty, and when that happens and he attempts to hump your older buck, all hell can break loose. And as Blue Eyes mentioned, this can happen when you aren't right there to immediately intervene. Which outcome could at minimum, result in serious injuries and an expensive vet bill. At worst, fatal injuries. To me, in almost all instances, it's just not worth the risk.
(A post from an old thread)
"Just to throw my two cents in, I volunteered (heh, before turning them in to theSPCA due to the conditions there)at a place that - albiet not formally - did "colony breeding" in the sense of having large runs of mixed sex rabbits. While the majority of the rabbits co-existed fairly peacefully, the few that for whatever reason either sought fights more or were picked on more had disastrous outcomes. There were ALWAYS a few bucks and does with terribly shredded ears, noses, etc. and on more than one occasion, the fighting was bad enough that rabbits lost eyes. Does seemed to fight more often, but bucks fought more viciously. I have been lerry of housing unaltered bucks together ever since, having seen the damage they can abruptly inflict on one another after coexisting peacefully for months."
If you decide to continue regardless of the risk, please keep a very close watch on the baby rabbit as he matures. In a few weeks time he's likely to start showing signs of puberty starting, by attempting to hump your older male. If your older male starts getting aggitated or scared of your baby bun pestering him with the humping, this is when the fights could break out, and when separating is likely going to become necessary, before a real fight breaks out and injuries occur.
If you decide to not risk it but would still like them to be able to interact, if they seem to get along fine for now, strictly supervised time together is a possibility. And having adjoining living spaces where they can still be near each other, but in the safety of their own pen (make sure the baby bun can't scale the fencing to your older buns pen), can be a way to help try and maintain a relationship between them without the risk of having them living together in the same space, until baby bun is neutered and you can then officially bond them.
This is what I did for the baby buns I raised from a doe I got that ended up being pregnant. She had 5 babies, and when they hit puberty I had to separate the boys, but each day I would sit with them in a room to let them continue to run around and 'play' together. I would have to intervene if any humping occurred, to make sure nothing escalated. And once they got to the point that the hormones were driving them to only think about humping the whole time, playtimes stopped happening, until I could get them all neutered and properly bonded in their family group.
https://wabbitwiki.com/wiki/Spaying_and_neutering_rabbits
https://wabbitwiki.com/wiki/Choosing_a_rabbit_veterinarian
they ended up just eating hay next to each other. They were together for about 30 minutes in a neutral area. We will do the same tomorrow, since it went well. Sunny will get neutered once he is old enough.
