Many people with CF have bacteria, including Pseudomonas aeruginosa, colonizing (living in) their lungs permanently. This is not unusual. Just almost everyone has bacteria (Staph aureus & Staph epi) living on their skin. However, most healthy people do not have bacteria in the lungs.
Pseudomonas is just a type of bacteria that your son has acquired in his lungs. Unfortunately, Psuedomonas (like any other bacteria colonizing the respiratory tract) can cause inflammation in the lungs & sometimes outright pneumonia. Other times, your son will be asymptomatic. Pseudomonas in the lungs is an extremely common finding in patients with CF, as is other bacteria.
The problem lies in that when the Pseudomonas becomes strong, then it can cause infection (pneumonia) versus colonization (livining in the lungs). Hospitalization will depend on the treating pediatrician - whether or not they think this the Pseudomonas has become strong (causing infection/pneumonia or a CF flare or exacerbation) or a colonization (relatively normal in patients with CF). To be honest, with an increase in cough & the onset of a fever, it sounds as if there might be an infection, which needs to be treated.
The major concern with Pseudomonas is that it can become resistant (not work well) against many antibiotics that are available. The doctor will almost always use two antibiotics to help cure the infection. I hope this helps!
~Larissa~