Final answer:
After a child victim becomes unresponsive while performing abdominal thrusts, shout for help, call for emergency medical care, check for breathing and a pulse, and start CPR if needed.
Step-by-step explanation:
When a 9-year-old boy who you're performing abdominal thrusts on suddenly becomes unresponsive, the next appropriate action to take, after shouting for help, would be to call for emergency medical care immediately. Then, proceed to check for breathing and a pulse and begin CPR if the child is not breathing. Remember, always ensure the safety of the environment before administering CPR and if necessary, move the child to a safe location before starting.