You don't have a good case

The traffic court is a very "fair" court. The police does not need to show evidence to prove your son did not wear seat belt. He just need to say "I saw the passenger did not have the seat belt on in the vehicle" That is already an evidence for them to place a charge on you cause the police is the witness on their side. You can pray the cop no show in court or maybe he die before the court day.