My Three Year Old Grandson Had A Bout Of Bronchitis

My three year old grandson had a bout of bronchitis in late Nov 2015 was giving a round of antibiotics and nebuliser treatment. He also had a fever. After the treatment his condition improved however a few weeks later he caught another cold and now has a lingering cough for about 3 weeks now. Dr said there was a little fluid at the bottom of his lungs, put him on another round of antibiotics and nebuliser treatment what could be the cause of this