My 84 Year Old Father Has Had Fluid Build Up

My 84 year old father has had fluid build up around his left lung only, started six months ago. Three hospitalizations, positive PET for lymph nodes 4R, 4L, and 7. Biopsy was done, five days later we were told the cells are cancer but pathology has not been able to identify what type.
The fluid production continues to increase, chest tube drains about 700-800 ml every other day.
What now?