Golden Cuckoo.
One thing I forgot to mention, and that you can't really see in the photos, is that its got a bit of white on the wings.
Do Marans or Australorps have white there?
Well, he's certainly not cuckoo.

Any black chicken might have a few white feathers on it's wings in juvenile plumage. My Australorps often do, so did one of my Langshans, and a lot of the Austra-Wyan-Orp crosses.

You'll get a better idea of his mature coloration somewhere between 12 and 24 weeks more-or-less.