Take her to a new dr to get answers.
Maybe try running a hot shower to steam up the bathroom and read books and hang out with him while he breathes in the moist air to break up mucus?
Give him lots of water to drink.

My daughter she had a cough for mths and it’s only at night 🤷🏼‍♀️

Mine hasn't slept in 3 whole nights or napped this cough is bad and nothing is giving relief we are both tired and I have no idea how to help him

My LO gets a terrible cough from reflux if she isn’t propped up properly before bed. (The acid flows back up from her stomach) so I make sure her head is elevated after she falls asleep..that combined with Matys All natural chest rub. 🙌🏼