State negligence laws differ but most all states require a driver to not follow too closely and to maintain a safe distance so that a driver has time to stop without hitting the car in front of them.
Insurance companies investigate an accident and determine fault based on driver statements, police reports, state laws, the state definition of negligence, etc.
From the account you gave of the accident it would appear that your son's car (vehicle #3) would at least be responsible for car #2's damages. If car #2 was only hit into car #1 after your son's car hit them then depending upon state laws he may be responsible for both vehicles' damages.
Though the first car may have stopped for no apparent reason the cars behind that driver should have left enough stopping distance to stop without striking the vehicle in front of them, taking reasonable care as drivers.
If the first car had stopped due to a child or animal running out into the street the cars behind would have needed the same stopping distance. Or if the person's car had broken down and stopped running the cars behind should have maintained a safe distance so they would be able to stop. Thus the reason for the first car stopping may not be as important to the insurance company involved as the reason that the other cars were unable to stop without colliding.
Free Auto Insurance Quotes
[Let CarInsurance.com help you find affordable auto insurance now.] To find out how more about how the insurance company determined fault contact the insurance adjuster assigned to the claim. For information on your state's negligence laws try contacting your local Department of Motor Vehicles. | 
