Transferring a car to one child while father still living

I am the trustee of my father's trust. He has offered his car to one of the siblings for him to trade it in for a car more appropriate for his needs. The value of the car would come from his eventual portion of the inheritance.

Need advice on the best way to handle this situation to determine the correct value of the car. Should it be based on the Kelley Blue Book value, the trade in value or what? Should we transfer the title of the car to the sibling before he purchases a new car, or accomplish the transfer at the purchase of the new car.

Advice is appreciated.