Yea an ER doc can check to make Sure he hasnt developed a PE (blood clot in the lungs) which is common and cancer and causes shortness of breath. The CT will also see if the cancer is bigger. Just be sure to go to an ED thats affiliated with his oncologist so they can see the images
He definitely needs to be checked out sooner rather than later, so if the oncologists cant see him urgently then he should still see primary care, or failing that, go to ED where they can check him over and perhaps even do a CT to establish whats going on with his nodes / progression.