That is not an easy task... to get them to understand that what they are doing is not helpful. They are confused and want to be somewhere different. Even if you could explain so that he understood it would slip right away as most current memories do. He doesn't know the difference between clean and dirty clothes. My Mom did this to an extreme. At least he's not packing the ice cream in the suit case
Having said this, you find ways to let him satisfy his needs (to be ready to escape) while making it easier on you and the staff. If his dirty clothes were put in a place where he did not have access to them then he could not pack them with his clean clothes. Could the staff remove them when he changes and put them in a basket or laundry bag in another room? Then it would make no difference if he packed his clothes or not because they would all be clean.
As for explaining to him that he needs to stay there, it's probably not going to happen. You can validate his need to be somewhere else and divert the time into the future but you can not reason with him and he will not remember. If it is only packing occasionally and verbalization then you can just validate, divert, and distract.... over and over and over. He has no idea that you are using the same technique over and over just as he doesn't know he is asking the same question over and over. If his desires are accompanied with anxiety, angst, or distress... then he may need medication to calm his anxiety. But even with the medication they questions remain.
Mom packed hours every day even moving furniture out of her room and throwing her belongings over the courtyard fence. Any attempt to divert her was met with resistance, anger, and combativeness. She was an emotional mess, crying and combative. She did spend 10 days in a Senior Behavior Med Unit and is now on a cocktail of anti psychotics and anti depressants. She is for the most part content and smiles often now... but she still ask, years later, about going home.
If we are not where we want to be, if we are in a place that is confusing to us, we want to go where we think we want to be. Since the confusion is in their minds then a move is truly not the answer but you can't make them believe that. They know what they know, in the moment, and it is just as real to them as our reality is to us. They want to be somewhere other than where they are... and that doesn't change. We can only find a way to make them content most of the time. So don't worry too much about finding 50 ways to say the same thing... just keep saying it as you have been. If you see emotional distress then get him help