How sad.
I hope the dog will be ok . You did all you could in the situation. If it was the mentally chalenged boy , then the adults who are in charge of that kid will be held liable for his behavior.
My son is 17 , autistic, and functions at a much younger age. He would never think to harm a animal. He is verbal , and goes to a special class in high school. My point is that if it was the same kid going around the neighborhood doing these things , then he is not being supervised at all , and causing a lot of trouble and commiting crimes . Now that the police is involved that kid may or may not be responsible.
I am very glad that you did what you did , and the dog was returned to its owner.