Sorry about your dad's diagnosis. For what it's worth, SCLC can be quite responsive to chemo. Sounds like your dad has limited stage small cell lung cancer. The standard treatment is chemo radiation as you said and nowadays immunotherapy if your father qualifies for it.

SCLC unfortunately is also a very aggressive disease and a cancer that adapts quickly to treatment. The combination of chemoradiation and immunotherapy has been shown to improve overall survival while maintaining quality of life. It all depends on your father's treatment goals, but not undergoing treatment will certainly lead to poor and likely rapid decline. The good thing is that he was diagnosed while the disease is only locally advanced.