What Stage was your Dad's cancer? If it was stage three, then the cancer may have spread beyond the immediate area of the rectum. If it was stage four, then it has spread beyond the area of the rectum. In either case, the chemo should help kill off any small, undetected mets.
I was stage three in 2001-2002 and had a similar treatment course recommended. I was only 45 at the time, so I thought it would help insure long term survival. So far so good. The chemo I had was called 5FU plus Leucovorin. It wasn't too debilitating for me. I was able to work through the six months postop chemo.
How old is you dad? How well did he tolerate the radiation and surgery? It's really his choice, but if he is willing and able to endure six more months of treatment, it should improve his prognosis.
Best wishes,
Lou