First off, I am so so sorry for you and your family. My thoughts are with you all as you go through this.
Although I’ve never had to deal with the life support issue, I have had experience with the loss of a loved one that we knew was coming. In January 2012, halfway through my junior year of college (I lived about 3.5 hours away), my grandfather took a turn for the worst with his cancer. He went into the hospital the day I went back to school after winter break, and about a week later my mom called to tell me that the doctors gave him a month to live. My 21st birthday was at the end of the month, so I planned to come home around then, but just a few days after the first phone call, I got another saying that it would most likely happen in just a few days. So although we didn’t have a set date like you do, we all went into prep mode. For the past 13 years, my grandparents have lived in the in-law suite of our house, so I was especially close with them. My entire family set up camp at my house, and the grandchildren (ranging in age from 3 to 26) came to say goodbye. However, I was still at school, so coming home was not so easy because of class and projects and all that. After many hours discussing it, and many tears, it was decided that I would say my goodbyes over the phone, because my grandfather would never have wanted me to leave school to watch him die in my own home. So one day when he was partially coherent, with all my family around him, I said quadriplegic goodbye to my grandfather, and bawled my eyes out. I later found out that when he was talking to me, it was the most “with it” he had been in days, and that made me feel so good. It was about three days after that, a Thursday morning, and I was on my way home (I had no Friday classes and just decided that I had to skip one day to go home a little sooner), when I got the call that he had passed late the evening before. Oddly enough, I barely cried at that moment. By that point I had accepted the reality, and honestly felt at peace because not only was he free from the pain and suffering, but because my family, who had been by his bedside day and night, had closure.
So although there is nothing that can truly take away the sadness and pain you will all be feeling, you are now given an opportunity that far too few never get, and that is to say goodbye. Take this chance to be together, and tell him that you love him and will see him again someday (if you believe in heaven and all). Know that, although it may not seem like it, he will know you are there, and you can now comfort him and ease him out of this life with love and dignity.
I am truly sorry for your loss, and wish you all love and comfort during this difficult time.