My 84 yr old FIL insists he's fine to drive 1900 miles to FL. We strongly disagree. How can we limit his driving (no long distances)?

They have driven back & forth for 15+ years, despite his wife's fear of an accident and requests to fly. His health & mental sharpness is decling some. Slower reflexes, difficulty seeing over the car's big hood & trunk, extra slow left turns-not yielding, getting lost at night, and instances of swerving onto the shoulders are some of the issues. We are not asking he give up his keys, simply limit driving to slower roads, short distances, and daytime only. He says no, even though my husband is willing to drive them back. He is a very strong willed, prideful man. What can we do?