My best guess is black faced. He looks a little paler overall than a regular grey zebra cock, and his black teardrop and breast bar markings don't seem as jet-black as they would be on a regular grey... I am not quite sure how to explain that, but he may have another mutation going on as well.
The black face mutation will cause the area between the beak and the teardrop to be black (instead of white). The black face mutation normally also causes the breast bar to appear to extend down towards the vent of the bird, often causing the upper portion of the belly to turn black as well. Your bird does not exhibit this second characteristic, which is a little bizarre, but could possibly be explained if he is just a poorly marked black-face.
The other possibility is that he has a mutation which I have not seen before. I personally haven't bred a lot of the mutations recently, not to mention zebra finches are mutating all the time. You might find the most answers by asking someone who has more recent experience breeding various zebra mutations:
http://pets.groups.yahoo.com/group/zebrafinch/
http://zebrafinch.info/
for starters
): http://www.flickr.com/photos/xomissi/932008326/in/set-72157600043968215/
That makes sense though!