I purchased a vehicle from a dealership, the son of the owner signed?
This pertains to you if you're in the US. So, you were at a dealership, bought a car at the dealership, the dealership owner's son was at the dealership, and acted as a representative of the dealership, and he signed the sales agreement. Yet they say he is not an employee, and had not the authority. Did you make a payment to him, or are making payments? You have copies of the agreement as well as receipt for the sale, I presume. I would visit your local office of the Attorney General and see what they have to say.