Try not to feel guilty. My grandmother in law spent the first few years at her nursing home telling us that her son John was coming to pick her up and take her back home (interstate). We just went along with the story, and asked when he was picking her up. We'd ask her if she'd packed yet, and how long was she going to Melbourne for. Then we'd change the subject to where she worked in melbourne, and which building was it. She didn't get angry or upset, but she was in her element taking herself back a few decades and telling us all about it. For a few years we'd go over and over this same story each time, about john coming to take her home, then changing the subject to something else that happened in the past melbourne to get her off the subject of going there.
We have arranged that John will be taking her home for her funeral in her home town when the time comes, because it's all she has wanted, to go home. It just took a lot longer for it to happen than she had envisaged.
Depends how well he will remember what you tell him. If you tell him he's staying for a couple of weeks, and then someone is coming to take him home (that someone is best an invisible who doesn't visit that regularly). Will he forget and then next week you can say, yes so and so is coming to pick you up after you have a 2 week holiday here, keep saying it over and over if it calms him. Gives him something to look forward to. I guess it depends if it calms him or winds him up that the 2 weeks is never ending.