Neutering a buck is an easy not very invasive procedure so it can be done early (contrarily to spaying a female, where most vets want the doe to be fully grown, around 7-8months, before proceeding). The only concern is the anesthesia. To reduce the risk (today, the risk is around 0.2 to 0.5 % if done in good conditions by a good vet) vets take the size of the buck in consideration. A bigger breed, which can be more than one kilo around 3 month old will be able to be neutered around 4 months old without any problem. For smaller breeds like nethies, they sometimes want to wait if the buck is on the smallish side. So, it really all depends on your rabbit. But 5 month old should be acceptable if your rabbit is healthy and has a decent weight.