Youre lucky that nile monitors dont have as serrated teeth as some of the indo species, this will be one of the many bites you will encounter with your nile. Leave it alone and make sure he learns to trust that you will not be a predator in his eyes, refrain from picking it up and such. If you have to do maintnance around his enclosure do it when hes in a hide or at night, that way he gets use to you when he is in a calmer state of mind. This is how I got some of my shyer and aggressive monitors to relax a bit.