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This could possibly be viral infection which will go away in a week's time. If he is lethargic, not feeding properly then he needs to be taken to the Paediatrician. Give him plenty of fluids like water, lemon juice. Keep him warm and use wet cloth to keep his body from rising to high temperature, also give Calpol
dose according to body weight. If there is no improvement he may need to be shown to Doctor.