My father has just found out that he has colon cancer which has spread to the liver and is not operable. What else can he be offered in terms of therapy?

Unfortunately, the choices for your father are very limited. For colon cancer which has spread to the liver, the prognosis is very poor. 

Perhaps you should speak to your doctor about hospice or palliative care. At this stage, most doctors only treat the pain and try to make life comfortable. 

The reality is that once colon cancer spreads, all treatments are futile. Life expectancy is short. Do not waste your hard earned money on charlatans who make false promises. 

Take each day as it comes and enjoy whatever days you have remaining with him.