The problem your having/going to have is his age. He wont really listen to family members, I was there a few years ago and it used to really annoy me when a family member tried to tell me what to do. Its just an independance thing.
If he is a law abiding person then stating the law might work, failing that hint at the costs if he should cause an accident. Sounds harsh but you need to make him aware of the direct effect it will have on him and not talk about the Dog. Most people react much more if you make it clear how much they will be affected as opposed to how much you or the dog will be effected...its human nature I am afraid. Make it clear any costs will be his to pay and if somebody was to die as a result then that would also be his fault.

Failing that you could always threaten to castrate him if anything happens to the dog