its not really a rule, but there are a few reasons as to why its recommended.
buns tend to do best in opposite sex pairs. if you had opposite sex pairs and they both aren't neutered, that would result in pregnancy. now, same sex pairs can work, but usually there is a lot more aggression/fighting in the bonding process, and there has people say that they're more likely to fall out or fight as a pair. generally, differing sex just seems to work better.
along with that, when they aren't neutered they're full of hormones. unneutered does and bucks are much more likely to be territorial making the bonding process a lot harder, or making fallouts happen among the pair. they're also likely to be more aggressive, sparking in fights in a bonded pair too. its generally harder to bond unneutered rabbits together, and you have a risk of falling out.
you also need to considered the neutering process, they should go together to the vet so that their bond does not break. however the entire process can put strain on their relationship, and I've heard of some falling out due to them smelling funny or just the general atmosphere (or perhaps the vets didn't keep them together, its impossible to know). the stress of a new bond and the vets could be too much also.
bucks can be neutered as soon as their balls drop, which shouldn't be too long, usually under 6 months.
theres a lot of variables and its a lot harder to bond unneutered rabbits, even to one that is neutered themselves. the unneutered rabbit is like to be more aggressive, and territorial to the neutered rabbit.
its just plenty easier to wait IMHO
he will be fine until then I would say.