Other than the normal resp assessment. Auscultation, inspection, depth and rate of inspiration, vital signs etc...I would want to know what his x-rays, verbal collaboration with pt, etc showed. Does he have underlying process such as COPD, restrictive lung disease, emphysema, etc from long term cigarette use? Is he barrell/flail chested, pale nail beds/clubbing. Is he poorly nurioushed? Any diaphramatic injury?
Inspect for any new, larging bruising on his chest, poor lung expansion.
Ask to possibly decrease the PCA amount as to not mask any new findings that the pt might not be able to verbalize under the effects of pain med.
continually monitor o2 sats, O2 requirements, pt's color, resp rate for possible resp decompensation.
Assess lung sounds frequently to assess for a pneumo secondary to rib fractures possible internal bleeding from seat belt injury.
Listen to the pt and how he is feeling. Pt's that say they simply "feel funny" are the ones to overly, overly, assess and get new films, CT's, etc. on. They usually have something happening.
continuous monitor, vital signs of course. They all give clues to possible blood loss from internal injury, resp distress, pain. All which will effect breathing.