
It sounds like he is bored or anxious when you are not there. I had a wall chewer too, same breed as Gemini and Sue - she has stopped now though as she is almost 6 :-D, must be something about Bernese and plaster

. I've also had one that chewed books, walls, facings, skirtings, metal blinds and chewed his way out of a metal cage - had to be seen to be believed! This was a medium sized breed too - we stopped it by letting him lie at the door we left by, as long as he was there he was fine, lock him in any other room and you came home to devastation. My setter adores plastic bottles, has been said - empty bottle, take off lid and plastic ring and let them play - Treacle doesn't even need treats in hers, she just loves to chew and pounce on it!
Good luck, it can be soul destroyiing when you come home to yet another chewed mess, but remember, he isn't doing it to get at you, he is simply finding something to do - maybe it started because he was anxious and now it has become a habit, breaking the habit might take a while but you can do it.