Your son appears to have croup whch is usually caused by a virus infection at this age. Prelone is a good choice of treatment as this prevents the vocal cords from swelling and causing breathing difficulties.It is not necessary and indeed contraindicated to give a cough mixture.He should recover after 3 or 4 days.
I find it difficult to advise you about your son's rash without being able to see him. It does not sound like eczema from your description. It may be a condition called seborrhoea or urticaria. The latter is more likely and is an allergic rash which closely fits the description you have given here. It would be best to consult a dermatologist about this rash.
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