What is the most likely diagnosis for Mr. Santos?

Mr. Santos, a 55-year-old male, presents to the emergency department with a productive cough, fever and difficulty breathing. He reports feeling weak and fatigued. Upon assessment, you note decreased breath sounds in the lower lobes, crackles and increased respiratory rate. His vital signs are as follows : temperature 38.5 °C, heart rate 100 bpm, respiratory rate 28 breaths per minute and oxygen saturation 92% on Room Air.