Since you have seen a doctor and your baby is not better. Kindly go back for a follow up with the doctor or see a paediatrician.
Meanwhile, focus on prevention. Keep the baby warm, no night baths or frequent baths. Avoid exposure to siblings and adults with cold, also avoid exposure to too much
A/C or direct fan. Avoid smokes, insecticides, fumes and all forms of air pollution.
Most cold are viral infections and resolve spontaneously on their own.