I hope you can get him there.. you must be really worried now and that's made worse when no one seems to be listening!
My Cousin will continue to let my Dad take him to and from work for as long as he possibly can (even though he's admitted he doesn't feel safe). The reason being, and I quote, "it would be four hours travelling time a day otherwise"
If your dad ignores the over 75 letter his licence will expire and his insurance will be invalid so he won't be able to drive legally anymore, this might solve the problem.