Advanced colon cancer

My dad has had advanced colon cancer for approximately18 months now and has had numerous rounds of chemotherapy and radiotherapy. It has spread to his lungs, lymph nodes and liver and is now having constant side effects such as bowel blockages, pain in lymph nodes and lungs, although he’s having treatment again the side effects have  seemed to come on quite suddenly with no relief from the things that worked before including bowel blockages. He has a McMillan nurse who is looking at other ways to make him comfortable (so I assume stronger pain killers) but I can’t help but think it’s coming to the end now? Apologies if this is an uncomfortable question but how did you know when your loved one was starting to slip? I’m just trying to prepare myself and think of how to prepare my son who is 4 and loves him dearly and doesn’t know he is poorly yet as up to now although he has deteriorated but uts been quite slowly and he’s been 90% functioning which has been a blessing, it’s only when looking back at pictures/videos etc you realise how much. Thank you xx