I am thinking of disabling his truck. My husband did not get lost despite his dementia until now. I cannot take away his keys without a big rukus. So I am thinking of having someone come by and disabling his truck. That way he will still have his keys and his truck, but he can go nowhere with it. How have others solved this issue?
So if I were you I'd try to get the doctor to explain it to your husband and report it to DMV.
But not all medical providers see this as their responsibility. In that case you need to do whatever you can, straightforward or sneaky, to stop him from driving. Getting lost is the absolute least of the worries. A person who can forget where the drugstore is can also forget where the brake pedal is, or that it is necessary to get a good look behind him before backing up. And generally the cognitive skills are worst in an emergency situation.
Whatever you have to do, prevent your husband from driving. He is not merely risking getting lost. He is putting other people's lives in danger.
The other question is whether he is safe to be driving at all, even with a nagivator by his side. A specialized OT behind the wheel evaluation can help answer that too. Besides getting lost, is he also driving erratically or have minor accidents or unexplained dings, dents and scrapes? Someone like that who should have been stopped from driving once hit my son's brand new vehicle and did not even notice that it happened - nothing was severely damanged, but I felt bad he did not know to get the license number and make a report that would have kept others safer on the roads...