May I suggest that you tell the son that you feel sorry for the chickens trapped in the shed and suggest that they let them out, or go over there yourself and let them out. Then when they fly onto the fence or into your garden, YOU clip their wings and send them home. Problem solved. They arent going to notice the clipped wings and eventually you wont have any more visitors. Good luck to you, I know it isnt an easy situation. I have neighbors that dont treat their dogs properly. I have had to step in a few times but they seem to take it well.