End stage Parkinson's -- At what point do we just let him pass in peace?

My father will be 93 years old in several months. He was diagnosed with Parkinson's at age 76. I started taking care of him and my mother in 2007 in my home. My mother passed away in 2009. I had to finally put my father in a nursing home because I was in over my head and felt I was providing a dis-service for him. The past few years have definitely been a continuous decline in his abilities. If you read the list of end stage Parkinson's disease, he is showing all of them. I have had him on palliative care. I can see that his quality of life is poor. He never made a living will. I have power of attorney for his health care. He was diagnosed with a urinary tract and kidney infection yesterday. The doctor is giving him bactrum and a probiotic. I'm struggling with this form of treatment at this stage. Yes, the antibiotic will help his infection, but will it really increase the quality of his life? My mind tells me "no". My heart has a hard time saying the words," no more heroic attempts to save his life". At what point do we just say leave him alone and let him pass in peace?