Hi Ozark,
You Wrote "how do i stop this behavior, before it gets out of hand"
Your Boys will need to find another place to live
All kidding aside, you may have hit the Nail on the Head. Your Grey could also be a little nervous around them now that he is more aware of what's going on around him and Boys that age have a tendency to move around rather quickly and sometimes have their mouths and arms flapping at the same time :-)
You can not stop a Grey or any other Parrot from deciding it prefers one over another. If the can feed your Grey or give him snacks and he accepts them as you say. It sounds more like your Grey wants to be left alone at times by your boys and lets them know so.
Perhaps have your sons start paying more attention to your Greys body language and if he shows he is not receptive to being handled or moved, just have them back off and go do something else for a while. :-)
These are just some suggestions based on assumptions, not knowing the entire story or facts.