Marie-Jeanne

The TX and its effects are very hard. This is a decision that only your father should make for himself, not the doctors.
How is your father's health otherwise? The Social Security Actuarial Life Table shows 6 and half years left for the average 83 year old male
ssa life tables Is it likely that the TX would give him more years than that if he is already frail? If he truly feels "his time has come", it's understandable to me that he would not want to spend a year of the time he has left in pain and speech therapy and risk losing his 'gift of gab".
I asked my doctor the same question after my cancer came back at 61 and was told the same thing. For me the math was simple: I could be dead by 62 or risk the permanent side effects of losing my "gift of gab" and ability to eat or drink but gain another 10 to 15 to 19 years. As I have a son and wife who depend on me, it wasn't a difficult choice.
Your father's math though merits exploration of palliative options with hospice at the end. With enough pain medication, even this horrible cancer can be tolerable for the patient although hell on the family.
The good news is that your father's doctors are not discriminating against him on the basis of his age. There is no age limit on these TXs working.
Charm