It depends on why his oxygen level is low. If it is because of spread of the prostate cancer to his lungs, then the situation would appear to be more serious. If the low oxygen level is unrelated to his cancer, but caused for example by emphysema, that would offer a very different prognosis. If related to the prostate cancer, the prognosis would depend heavily on whether the spread to his lungs is occurring before or after cancer therapy.
Good luck.
My heart goes out to your family. I hope that your uncle is on continuous oxygen. It is the best thing to do for comfort, as well as support for healing. An improvement in his ability to hold onto oxygen, and keep his oxygen level above 92% even with activity, would be a sign for hope. You can't improve oxygenation by ignoring low oxygen level. The goal is to avoid ANY and ALL low oxygen levels, take 30-40 deep breaths/ day, or as many as the patient is able to do. If your uncle can't walk without causing low oxygen levels even with portable oxygen, then he should avoid activity. If he can maintain 92%, he should be as active as he can tolerate. Heart Failure Solutions addresses concerns for all of your uncles issues and can help you recognize danger and what can be done to support your loved one.