Since your son is 18, he can file a lawsuit against his attacker(s) for assault and battery. Assault and battery are both criminal and civil.  The fact that nothing happened with the criminal case should not deter him from filing the civil case (lawsuit) against his attackers.  His damages (monetary compensation he is seeking in the lawsuit) include compensation for his dental bills, pain and suffering (an amount in addition to the dental bills), and wage loss if applicable.  He can also seek punitive damages ( a substantial amount) to punish the wrongful actions of his attackers.