I know some very poor quality, dark mojaves are said to be easily mistaken for normals as adults, so I expect he is just not a very good mojave. I've never personally seen that before, though. (You never can be sure what will pop out when you pair two snakes for the first time).

I had some lessers the year before last that people swore up and down had to be mojaves--they looked almost exactly like them, they were so dark. (I'm going to experiment with that, so I kept a dark one in 2010, but I won't be doing that pairing again, lol).