There is cancer cells in his left lung but pathology can't pen down the cause or where the cancer is?

We can't help. Pathology is usually helpful in identifying a potential source for metastatic disease, but as you are finding out, sometimes not conclusive. Further testing including imaging may be done. Ultimately, the physicians on the case know much much more than we do and it is hard to offer any additional insight. Be persistent in opening lines of communication with the treatment team.