Maybe he's socially awkward and his therapist is encouraging him to try to interact with people?
Anyhow I wouldn't worry that he will tell people you have OCD. For a security guard, the kind of attention to detail and precision that comes with mild OCD is an asset.
You can just tell him, "It's part of my routine.", "Yes this is my favorite book" or "All day, every day." Just pick a favorite response and keep using that. Eventually he will get bored with the same interaction every day and even if he does not desist, you'll know exactly what to say and eventually it'll just be part of your work routine.