> He is walked 3 times a day, so he cannot be bored
It depends how much mental stimulation he needs. If his 3 walks a day include experiencing new places/people/situations and training then they will be providing
some mental stimulation, but at 8 months old each walk shouldn't be very long so that still leaves loads of hours in the day for him to need to be entertained in some way (even if each walk is a hour long that leaves 13 hours a day of doing nothing - factoring in 8 hrs of sleep per night).
Being that he has had the opportunaty to destry an entire sofa he must be being left to his own devices for long enough periods that lonliness/boredom/seperation anxiety/the need to chew or play are getting the better of him.
Pesonally, my pup was never left for more than 2 hrs per day without a dog-sitter and when he was left he was in a dog-proof room (the kitchen), after ensuring he had allready had some mental stimilation (training & play took care of the mental stimulation) and he was left with something to do (a cardboard box with treats hidden in for him to chew his way through).
ETA JG & suejaw beat me too it and without all the waffle I always manage to do!!