Ok so I definitely need to tell my mum, which will hopefully mean he won't be allowed to drive my brother anywhere when he's in his custody.
Don't just tell your mum, this has gone beyond telling on him, you are an adult, report him to social services, your brother is a vulnerable adult who's life is at risk.
As for reporting him, as I said we don't live anywhere near so I have no idea as to when he gets in his car. Do I hope that the restrictions to his care of my brother is enough to shock him into stopping?
Very unlikely. Your dad already knows that what he is doing is dangerous, what makes you think he will suddenly stop.? He is risking his own, your brothers and other road users lives regularly and doesnt care. (I dont say that unkindly, Im trying to get you to see he is not going to magically stop, it will only escalate as it has already, you yourself said you never thought he would do such a thing.)
Is it possible to report to police that he's a likely candidate for drink driving without knowing when? I don't know his number plate. He's retired so doesn't drive to work at routine times for example.
Report him to the police anyway and the DVLA, pull out all the stops to try and protect your brother, Doesnt matter if you don;t know his number plate, you know his address and date of birth presumably?
I understand you are facing difficult choices and you love your dad but please know that and I say this from first hand experience, lives most definitely are at risk because of his addiction.