Well, I could be mistaken. We are trained only slightly in xray and ultrasound, but from what I see and the more I look at all the structures of the baby... it almost looks like a hand between the legs. you can see a slight shadow of a line just above the nub... but who knows! lol I know from my degree that around the 13 week mark, the babies largest bone in the leg (femur) is only about 13mm long... so that's why I think that it could possibly be something else other then a penis. Id imagine that at 13 weeks a boys penis is only about 2 or 3 mm, compared to the femur bone. But... Having said that, Im sure your doctor will be able to confirm for you either way very soon!!
Congrats again!