it look like his balls, to be sure flip him on his back and have his feet pointing towards your knees grab the tail and push down gently on his tummy down there with your thumb, if a tube thing comes up it a penis if it's a slit its a girl.
Well it's just that we were told it was a girl when we got him/her so it was a little shocking to see when a friend asked to look so she could tell if her bunny was a boy or girl
If he was a baby when you got him, it's actually not uncommon for a rabbits sex to be misidentified, as with some bunnies it can be quite difficult to tell for sure which sex they are until fully matured. http://www.rabbitnetwork.org/articles/sexing.shtml