I would go with slippery elm (in porridge, water or Thayer's cough drops), licorice root or marshmallow root tea for soothing the throat/respiratory tract. I also use peppermint and eucalyptus essential oils in a cold air humidifier for the duration of the sickness.
Raspberry leaf and Elencampane (both infused in tea) work well for drying up mucous lining
Ginger will also help in getting rid of excess mucus.
Other good herbs for babes (or adults) with runny nose/cough/cold include:
A ginger tea with any of the other above herbs loaded with fresh lemon should help immensely. Echinacea can also be added but my thought is that it's best used during the first few days of a cold.
A little olive oil or herb salve with a drop of eucalyptus, pine or peppermint oil and rubbed under nose can both sooth the irritated area (from nose-blowing) and help clear the airways a bit.
He should not be having any dairy or sugar right now. Both those foods cause congestion.
Hope your son is well soon.