My dad has small cell lung cancer, after chemotherapy the tumor was reduced 80%, can it still grow if left alone?

Yes. Small cell lung cancer often responds very well to chemotherapy (shrinks) but it is unfortunately also common for the cancer to return after the completion of chemotherapy. Your medical and radiation oncologists can give you the best information for your father's particular case.
It can spread withou. It can spread without any therapy, it is not cured and you have to continue treatment recommended by your oncologist.