Your dad's doctor should be consulted ASAP.
It is his/her DUTY if a safety issue is involved to counsel your dad to give up his license or report the situation to the local State Authority (in MA. for example it would be the Registry of Motor Vehicles. BE AWARE SENIORS DO NOT WISH TO GIVE UP THEIR LICENSES IT IS A MATTER OF PRIDE AND DESIRE TO BE "INDEPENDENT" !! HANDLE WITH CARE!
Hope this helps!