The doctor says that its not mumps but my son's (3 years old) lymph node is swollen bec. of his coughs and observe it by 1 wk. is this just normal?

Yes it is normal: Lymph nodes are the place in the body where cells which fight infections gather to launch themselves against an infection. Upper respiratory viruses cause most common childhood illnesses, especially those that include cough. This is because the virus invades the nose and oropharynx (mouth and throat). The nearby lymph nodes (in the neck) become swollen while the body is battling the virus.