Hi! You know, to give you the best answer, I think I should ask you a few questions first that will help define the problem and the situation.
The OCD diagnosis needs to be clarified here as it is usually not treated in such a "secondary role" way as you have placed it. Does he have OCD? You wold know. Or is it that he just has some OCD "type symptoms" when the anxiety is very severe?
What do you mean that he thought he was entering a retreat? Did he not know that it was a hospital? Why would there need to be deception?
If you are seeking brief therapy for his disorder(s) because you live far from a city with psychologists and psychotherapists and have none available to you, would he consider seeing someone for the two weeks intensively and then continuing treatment by phone?
Any extra information that will help, feel free to share.
Let's go forward from the answers to these questions.