He's too young and wont understand. You said previously that this man is a father figure to your GS. So you will be putting him in between you and his dad which is unfair to do to a child. He will end up resenting you instead of the dad. It also shows him that that is how people talk to you which will reinforce that he can speak to you in the same manner. no good can come from this with a young kid. He may seem mature, may even be bigger than you in size but his mind is still that of a child and this will be very confusing to him. Instead of pulling yourself up in his eyes by putting this man down you should rise above all of them and show him how a mature respectful adult acts. Your bigger than this and it sounds like you may be the only one who can teach him how an adult should act.
Besides the fastest way to push a child to someone you dont want them to be around or act like is to bad talk them. I would just tell him that you understand he cares about this man and that is his right but you are having disagreements with him right now. Leave it at that - dont involve children in the business of adults.