Long continuing cold

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Hi,

My son (5 year old) has had a cold for 3 and a half weeks now - we give
him lots of steam and this helps - it thins down his mucus. But during
night time, maybe its the position, his mucus thickens and he gets a
stuffy nose - which makes sleep very difficult. He either has a very
disturbed sleep or wakes up for hours. We are using a warm mist
humidifier in his room at night. Other than that and the steam, what
else can we do so he is more comfortable at night?

Are there any remedies that can help with this?

Thank you in advance!

Shanti