Ask him to sign an agreement that he is doing it with permission and that you can ask him to remove it within 30 days at any time. Since it sounds like you're on good terms you can tell him you don't have any plans to tell him to remove it, but you need the protection.
The reason to have something in writing that he has your permission, and he acknowledges it is that he then cannot claim any kind of adverse possession, which requires occupation that is "hostile" (ie., without permission.)