My dad was diagnosed with cancer over a month ago and we still don't know what kind?

A mediastinal mass was discovered during an MRI that my dad had as pre-testing for surgery. Given the amount of (severe and permanent nerve damage) pain he was in, we all agreed to wait until after his neurostimulator implant was put in to do a biopsy. Good thing, since his lung collapsed due to the biopsy.

The biopsy came back showing malignant but poorly differentiated cells. He's had a PET scan, tons of blood work and literally more ct scans than I can honestly remember. Just had one Monday and is having another tomorrow.

At first we thought it may be lymphoma but that was ruled out. That left lung cancer as our next guess.

The oncologist told us 2 days ago that he is diagnosing and treating this as lung cancer, even though the most recent ct report says no convincing primary lung lesion. The radiologist today said it is not lung cancer, there is no lung or lymph node involvement and to wait to see what the second opinion biopsy results are (which we hope to have next Wednesday).

This is all very frustrating, my dad is only getting worse each day. He is down to 98 pounds and he just wants to start chemo. He'd rather be sicker and taking action then sit and do nothing. We don't even have a stage or prognosis at this point, so he lays wondering at night how long he has.

Advice?