Ok, are we talking "nibbling" which does not include any breaking of the skin, or bleeding? Or full fledged biting where he's bleeding from the bites, or the skin is broken?
Rabbits are like cats -- they groom and groom and groom. If it's just nibbling, then I wouldn't worry about it unless it progresses to biting (in other words, just keep an eye on it).
If it's biting, then I'm not sure what to suggest -- especially if there are no other incidents like this to suggest fighting... Having never kept rabbits together myself, I'm not sure what proper protocol would be for this.