My son is a minor, he signed a “contract” that was written on paper from a guy who was of age is the “contract” valid?

My son is a minor, he signed a “contract” that was written on paper from a guy who was of age. The paper was written on paper with a pen, made at his house. They ended up making a vehicle trade my son’s quad for the guys truck, the guy is refusing to trade the truck for my sons quad back and the guy does not have the title to the quad in his name either. 1. Is that written agreement valid? 2. Will that agreement mean something to the police if I report it stolen and give them the guys address? 3. Sense my kid is under 18 does the “agreement mean anything anyway?