The best thing here would be to mix some warm water with honey and lemon as this is very soothing, plus it is sweet so you shouldn't have too much trouble persuading your son to try it!
Another option would be to try our Cough Spray which is suitable for children over the age of 4. This contains the herbs mullein and marshmallow which, again are quite soothing, and can help to ease dry, tickly coughs quickly.
Do you have questions? Please feel free to ask.
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