I think that you have just answered your own question You face this as a family united by this horrible disease.
You support your dad as much as you can, ensure that his pain meds are doing what they're supposed to do. I know that it is hard, but you try to look at this in a more positive light. Build up memories whilst you can. See if there are people or places that your dad would like to see, if he is still well enough. See that he is eating enough. This is all very basic, but he will be happy to just have you there with him.
Once you know that the inevitable is coming, you begin the grieving process and this is so hard to deal with. How is he coping with chemo and how many sessions is he due to have?
You will all find the strength you need to get through this. I have lost both of my parents, many family members and close friends to cancer and have had 2 bouts of it myself in the past 8 years. It is not easy to cope with, but sadly, we just have to get on with it.
i just thought I'd pop my dads story here incase anyone's googling richters and comes across this as when I was there's wasn't many results.
my dad was disagnosed with CLL he had this for about 5 years then it transformed into Large diffused B cell lymphoma stage 4 when found, largest lymph node was 18cm, and his blood were not good at all.
he had 6 rounds of RCHOP, he found the chemo very manageable! No massive side effects! Abit of nerve tingling in his fingers, metallic taste in mouth and abit tired but he still got up every morning and walked the dog, pottered about through the day and ate every meal as normal.
He was officially in remission with good blood levels, CT scans showed NO active lymphoma after his chemo. He's no 26 months down the line from being diagnosed and 20months of being in remission, in 3 months he has his bloods and consultant appointment and if everything's still ok then he is classed as cured.
i just thought I'd leave this here to give hope to anyone googling and stumbling upon it. X
