Every illness even in the mildest form can progress or can take any shape, so answering that nothing to worry is not right, upper respiratory infection is commonest illness hoever they can progress to lower respiratory tract or chest over few days but most of the upper respiratory infections healed by themselve with help of supportive treatment so a check or monitoring on a child has to be done during even simple cough coryza. you can keep watch on his breathing rate , chest indrawing, activity, feeding at home. if the symptoms of fever and congestion of nose not improves by day 6 then antibiotics be needed to start but please ensure to get your child checked if it not resolves, persist or worsen.