See advice below.
Use saline nose drops and suction the mucus from the nose as often as needed (if he can blow his nose, do that after saline drops).
Also, elevate the head of the crib or bed. If your doctor ok's it, use a humidifier
in the room during sleep
. Cold and cough
meds are proven not to help and can cause severe side effects. If these actions don't help enough, he needs to see his doctor.