At 9 months old, a baby's cough is often caused by nasal congestion dripping down the throat, especially when lying down. Using saline solution to irrigate the sinuses is a good idea and can be used more than twice a day to prevent mucus buildup. Avoid giving the baby saline solution at least an hour before bedtime. Menthol-based ointment can be applied on the chest and back at bedtime, but not on the face. Generally, cough syrups are ineffective for babies. If you have any further questions, feel free to ask!