Many people say to introduce them on neutral territory-away from your home. Take them for a walk together. Why are they re homing him because they have a 2 year old? That sounds suspicious to me,as I had 5 dogs,with little kids,and no problems. If you have children, just be cautious. As for the plastic, I have heard of dogs and cats having a thing like that about plastic. My niece has a cat that will pee on anything plastic,especially like shopping bags. You can also buy spray,that is supposed to neutralize the scent,if he does mark. I (in my opinion) that if watched closely, he can be taught NOT to mark in the house. Good luck and hope it works out! You may need a bit of patience, @brutus77 is an expert!