Your dog may be doing this for a variety of reasons -
is he teething? Is he bored? Is he doing this when he is left alone or when you are there?
If he is bored, then only you can change his behaviour and make it different for him. Play games
with him to stimulate his imagination, such as hiding his toys and encouraging him to fetch/find them.
Don't make the hide too difficult to start with. Always give lots of praise when he finds the article for you.
If he is teething then there are various products on the market that can help. Personally I love the Kong -
a hollow cylindrical toy made of very hard rubber. Fill the Kong to the brim with moistened food and rub around
inside with highly scented treat such as pate (not too much as this is salty) peanut butter, liver paste or
cream cheese. Give this to your dog when you leave him and he will be so busy trying to extract the food that
he will forget about the furniture. If he is doing this chewing in front of you, then you can chastise him,
but don't think he knows he has done wrong when you come in and find your sofa shredded. He only knows that
when you return you are cross, he doesn't understand that chewing the sofa is wrong. A dog should not be given
free access to the whole of the house until he has fully outgrown the chewing stage, so you have to restrict
his area to one room, or a crate if you are not leaving him for too long. Hope this helps.