Hi Diane I am very sorry to hear about your Dad.
I can totally relate to what you're going through as I lost my dear Dad a week last Saturday to terminal colon cancer that had spread to the liver.
We only found out he had cancer about six week before he died as he hid all of his symptoms. First of all, his life expectancy was positive (18 months), then it went down to six, then three, then finally about six weeks.
The Tuesday before he passed, the GP came out and said my Dad had 'days' which was a shock as I had thought he had at least another 3 weeks. The main signs I saw were that he became more jaundice, and his appetite completely went - even though this had been reduced for months he had always managed small meals but all of a sudden he could barely manage a spoon full of cereal. He also became so weak that he didn't leave his bed and slept most of the time. He was always mentally sharp, but in the last few days he became a bit confused. A lot of weight dropped off him in a week so he we was basically skeletal. One big thing I noticed six days before he passed was that his pulse became very strong and noticeable on both his wrist and neck - apparently this is normal as the heart has to work harder . He also complained of a 'burning' sensation on his legs and the Marie Curie nurse said that this part of the process of his body shutting down.
However long your Dad has left, I would try to take each day as it comes. Unfortunately, everything becomes literally completely taken out of your control and you literally just have to learn to live with the unexpected. I know how hard it is but try to keep a bit of normality in your life as otherwise you could burn yourself out. Try make the most of every minute and count every day with your Dad as a bonus. He could surprise you and go on longer, or it could happen suddenly.
I wasn't prepared for how much the actual dying process would upset/distress me so much. I naively thought he would just 'pop off' but watching him on the end of life meds was very hard - make sure you have plenty of support if you are going to stay with him, and take regular breaks to help you stay strong.
Even though my Dad was virtually completely unreactive to anything, I do strongly believe he could hear what we were saying as he would occasionally life an eye brow so make sure you keep talking to him.
Big hugs and love to you and all your family.