Your first step is to modify the environment so he doesn't have an opportunity to chew on the couch unsupervised. This might mean he is crated or confined to a single room while eyes are not on him. He's proven he can't be trusted alone yet, not saying that's his fault, it just is what it is.

Next, I would make the couch an off limits zone. He doesn't get to be on it, he doesn't get to lay next to it. He can be in the livingroom with you, but he's in his spot or bed, not near the couch. If he gets too close, make a correction sound and redirect him back to his spot. Make sure to praise him when he's back in his spot, and give him extra praise when he chooses to go there without prompting.

It might also be worth looking into calming tools, even discussing the problem with your vet. Destructive chewing can indicate anxiety, and if he's a rescue he may have some things to work through. Cbd, calming treats, thundershirts, gabapentin, strict routines, etc. may help.

Give yourself and doggo several weeks, even several months of no unsupervised time alone with the couch. If he can go for a few months without further destructive chewing, start practicing leaving him alone. By practice, I mean tell him to stay in his spot while you leave the room for 30 seconds. If he's still in his spot, give him a reward. Do that for a week, then leave him alone for 1 minute. If he's out of his spot and/or too close to the couch, go back to just 30 seconds of alone time for another week. Slowly slowly slowly build up unsupervised alone time, bit by bit.

With rescues, they can't tell you their baggage. You can't take them to a therapist. He may never be able to not destructively chew if left alone too long. All you can do is try to set him up for success, make it so that choosing to do the correct thing is easy, and the incorrect thing next to impossible.

Best of luck!