
Hi,
I found the following section in a book by Dr E. Franklin. It is about dalmatians, but it can apply to any breed.
"The third class of difficult dog is also rare, but it is known to most exhibitors. It is the dog, often excellent in appearance and behaviour at home, which becomes completely bored in the ring. To rouse it from this gloomy stateis often impossible, and a ringsider might well imagine the dog to be ill-treated, so glum and apathetic does it appear. It hates a show, though once away it often becomes a different animal altogether. If a little experience does not correct this behaviour such a dog is better left at home."
It must be really frustrating for you as he's been so successful, and so nearly has his title, but if a long break from the ring doesn't renew his interest you may have to call it a day with him. :( But at least you know you're not the first who has had this happen, and you won't be the last.