YOU, yes, you are the one who is responsible for this.
You are negligent in several ways.
1: Never ever allow your son to meet a stranger at the curb.
YOU exposed your child to unknown risk.
2: You are liable for any charges on your credit card which your son may make, including and besides the pizza.
3: YOU are liable for any damages which may occur by allowing your dog to run loose.
Finally, it is a totally negative example which you are setting for your son by not having the courage and/or integrity to accept the blame for this entire incident which is rightfully yours and yours alone.
I have done many things in my long life for which I have been culpable and have ALWAYS felt better about myself when I admitted thiese things to myself.
Stand up and act like a responsible adult instead of attempting to get something from someone else who has NO responsibility in this matter.