Go on line and print out the news artical about that old chap that killed a motorcyclist recently, I think he got four years. He was basically too old to drive and ended up killing someone. Give the artical to your old man to read before he ends up in a similar situation.
Do something now, you may regret it later. His feelings will count for nothing if he kills / injures someone or himself.
Im sure he offered advice to you when you were young that you found hard to take, but it was the best advice.
Get in the car with him wearing all your body armour and some cushions strapped to you, and hand him a bag with 'airbag' written on it... I had the same issue with my grandad. in the end we just sat down with him and told him we where concerned about him and his safety. He had a bit of a grump and then came good a few days later. Worth a go...
