Talk with your family members. I would say that family has not just a right but even an opinion to intervene. However, you may be a bit too worried and you should go for unity in the family. That way you can also discuss measures that might compensate your grandfather loosing his license, such as making a more coordinated effort in seeing him regularly. Be careful, though, not to write him of too quickly. Again… talk to your family members and if you are the only one worrying, then you might consider listening to the rest. (We have been in a similar situation with our family, but that sorted itself out, since our grandfather opted out from driving himself.)