What can I do to get a 9 year old child's cough to calm down enough for him to go to sleep?

Find the cause. You need to know first what is causing him to be coughing that much. Asthma, post nasal drip, regurgiation can cause cough, let his doctor evaluate him.
Honey. Try 1/2 - 1 teaspoon of honey 20-30 minutes before bed. You can also use Vicks BabyRub on his chest and neck. A humidifier may help as well.
Depends upon... The effective remedy for your child's cough depends on the underlying cause. If your child is coughing often enough to interfere with sleep, have your child evaluated by primary care physician.