[sokel]

Not a route I want to go down, had he been humping cushions I probably would have but like you say your dog humped cushions  you reprimanded him he stopped humping cushions but still mated bitches, Not quite the same sinario here he is humping dogs, if he is reprimanded he may well stop mating dogs.
Like you I am a show/breeder . Its in my blood, spent my baby life at shows most weekends with parents and grandparents, started handling at 6 years old , showed my own dogs by the time I was 10 and made my1st  own home bred dog up to a champion by the time I was 12.
Now 51 and still as keen as I have always been

[doganjo]

I'm still not convinced you'll put a pup off by telling him 'no' - or even just a  distraction - stones in a plastic box thrown down near him when he starts?  I agree with Sandy on that, and the water in a spray - I have a water pistol handy in the lounge in case he starts on my nice silk cushions covers.      You could try valium 

It's a shame you can't ask his owner, hope they aren't away too long.

Our paths may have crossed in the show world, who knows - it's in my blood too.