I don't think you'll regret that decision. Also, there is no way he should be by himself. End-of-life care for my stepfather was EXTREMELY labor intensive. I had breast cancer last year and he helped to care for me through chemo and surgeries, etc. For me, it was wonderful to be able to give back to him in this way. I just wish he had had the outcome I did. My heart is still broken. He has left a HUGE void in our lives. But the fact that we both "shared" cancer, ended up being something that strengthened our already strong bond. I'm sure you will never second guess your decision about leaving early to be with him and will treasure the spent time with him for the rest of your life.
Your dad may be a very private person, as was my stepfather. He accepted that his death was imminent, but had a hard time "letting go" of us. He told me dying wasn't the hard part...that saying goodbye to the ones you love is the hard part. His final week was such a struggle. Hospice even brought in a harpist to try to help him pass over. We had finally, on what turned out to be his last night, hired a nurse to come sit with him overnight, so we could get some sleep. He passed away in the first 15 minutes that the nurse was alone with him. I have a feeling it was easier for him to "let go" once we weren't in the room.
Good luck to you and your family in the days, weeks and months ahead.
Trice