Well, it looks like he has the following breeds in black: Java, Jersey Giant, Wyandotte, Cochin, Langshan, Crevecoeur, La Fleche, Australorp, Ameraucana, Sumatra. If he isn't a mix I'd think you can eliminate everything except the Java, Australorp and the Jersey Giant. So he could be a lorp with yellow instead of white skin, or one of the other two with a lot yellower legs than they usually have. Mottled Java have legs more like his, I'd wonder if he is a cross between a black and a mottled and didn't get good leg color? He just doesn't look hefty enough for a giant even at four months, but I guess if he hits 12+ pounds as an adult he could be that.