diabetic father could go blind soon but doesn't care

I am 16 and my elderly father, who is diabetic, recently (well over a month or two ago) got told by the doctor that he will go blind in 3 months if he doesn't look after his health better. His main issue is drinking many fizzy/ sugary drinks daily. The doctor also made it very clear that this is his last warning.
Me and my sister have literally screamed, begged and cried at him to stop because he won't be able to meet his future grandchildren, attend our weddings, someone will have to stay home all day just to look after him, etc. and he kept saying okay to us and saying that he will stop.
He started doing it again anyway by hiding drinks from us or drinking them not Infront of us and just blatantly drinking them In front of us. Absolutely NOTHING will make him stop- no amount of screaming, crying, begging or telling him how it will affect his life.
We genuinely may as well talk to a wall then to him as he will not stop or listen to our pleas. It is so so infuriating and heartbreaking at the same time. I don't want this to happen to him obviously, but he is not listening to us at all.
Does anyone have any advice? I would appreciate it a lot.