Hi Jenny,
I am from Spain. I hope I can help with some tips and ideas.
How about:
1)-putting a sign on the front door, from the inside, saying something like: "VIRUS ALERT: government says we must all stay home. Cannot go outside". With a warning sign. To help him understand, use simple language, few words, visual aids. That could help him remember he must not go out, when he is about to leave.
2)-does he usually read the newspaper? you could leave a newspaper showing a page where it says that the government is advising people to stay at home because of the virus in a table where he usually has breakfast, lunch, dinner, and go frequently during the day.
3)-can you lock the door from the inside, and ask your mom to keep the keys? Then when he comes asking about the keys, if he cannot understand or seems to get upset, maybe try to redirect him to an activity or something that will make him forget he wanted to leave, e.g. "yes, I will go and look for the keys, but can you please help me with ____ while I look for them?"
4)-you must put safety first.As a last resort, you may have to prevent his or her access to a car. Some methods to do that include:Hiding the car keys, Moving the car out of sight.
5)-buy a system that will notify when the door is opened: there might be available some king of technology or bell that can notify your mom whenever your dad opens the door and is about to leave, so she can go and look after him.
Also, might help trying to think Why is he leaving:
- Does he need something from the supermarket (try to make sure he has everything he could need)?
-Is he bored (try to keep him engaged with an activity he enjoys)?
-Is he feeling lonely (can your mom take a break from what she is doing at that moment, when he wants to leave, and do an activity with him?)
-Does he need to move around? Can he and your mom take regular walks on the garden, or around the block if they leave in a place where there isn´t too much people around? Can they do some exercise together at home?
If he seems to be having difficulties understanding, maybe the best strategy will be to distract him whenever he wants to leave, and engage him in an activity that he enjoys and will keep him busy. I would try to avoid getting angry at him, or escalating the situation if he starts getting upset. If he does not understand, you could just breath, go with the flow, say "yes, you will leave but first please ____" and try to redirect him to something else.
Please let me know if any of these help. I will keep thinking and searching for strategies that other people have taken in this situation.
Best luck and lots of patience,
Natalia