Does he live with another rabbit, who could have bitten him? Is it swollen, sore, or warmer than the rest of his skin (might indicate infection)? Does he scratch at it? Sometimes rabbits have mites or other skin problems and you can only tell because they scratch their skin so hard they hurt themselves.
For now you can put Neosporin on it, not Neosporin Plus because the extra stuff in it is bad for the bun to lick off. If it's possible that another bunny or other animal did this to him, keep them separate. Depending on how it looks and feels you may want to take him in to a vet on Monday.