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Viral infections are known to cause systemic symptoms such as the ones your baby has. However if symptoms continue to persist for longer time, other causes of infection needs to be ruled out.
Currently I suggest you to continue feeding him well. Tylenol
pediatric preparations can be given as per his weight thrice or 4 times a day to reduce body temperature. Watch for more rashes and if it happens, a physical examination and investigations are warranted.
Hope this answers your query.