Congratulations on finding such an excellent trainer for Grimm. From your description, the fellow certainly seems to know what he's doing: he gave a very reasonable explanation of Grimm's behavior and explained a solution.
Our trainer here in Chicago offers an "away-from-home" obedience training course, with a similar set-up to the one you described. He's an animal behaviorist as well as a trainer; this education and experience give him special insight, of course. The course is either 2 weeks or 4 weeks, depending on the dog, of course, and the entire course concludes with some private lessons, during which he works with the dog's owners as dog handlers.
I would certainly recommend him to you. However, commuting to Chicago from Germany might be a tiny bit too costly--especially when you have found such an <u>excellent</u> trainer so much closer to you!
Good luck to Grimm as he goes off on a short trip to boarding school!