It could be that he is excited.. Or might mean he is agitated. My amazon clucks as she paces and feathers ruffled, wings out to look big, and eyes bright.. She is generally in a biting mood. I don't try to get her out, if she acts this way. But when she clucks during playing and gets under my shirt while I lay back in the recliner, well her neck feather ruffle and she is excited. Her wings are not to intimedate a preditor. It really is hard to tell the difference in moods- but one clucking means excited and the other is agitated.
I know its a different breed so I'm not sure, but my bird clucks at me when she is annoyed lol, if she is sitting quietly and I make a loud sound or talk to her when shes not interested she clucks! I actually love the clucks, so cute hahaha.
My grandmother has two Amazons- one yellow naped and blue fronted and they cluck when ever they are in the mood, esp. the YNA. They purr and fluff when excited or annoyed
Jake YNA 1970,Kia Panama amazon1975, both i removed from nest and left siblings, Forever Home to,Stacie (YN hen),Mickie (RLA male),Blinkie (YNA hen),Kong (Panama hen),Rescue Zons;Nitro,Echo,Rocky,Rub