My 5 and a half month old son has been congested for over a month now. The doctor tell us not to worry...?

He seems so sick. He runs a fever on and off and today he is coughing and breathing so hard! He cannot even sleep soundly because he is so stuffed up. I keep sucking stuff out of his nose, but he looks and sounds miserable! We have gone to the doctor every week with this issue and the gave him a nebulizer to use which seems to be making things worse. Has anyone else had an experience like this?! We have now seen three different doctors as well!