Is there any noisy breathing? If your child is having severe cough, get examined properly to rule out tonsillitis, pneumonia (both these cases he will require antibiotics mostly) and cruop (may require steroids). If everything is fine no need to worry, once the viral fever settles, child will become active. You can give any properly cooked,fresh food. Even though unlikely in this case, it's better to rule out foreign body aspiration.