
Poor boy, they're doing a good job with him considering the damage his first owners did.
I don't think they can fix the chewing unless they find a way of confining him to a safe area so they will need to work on crate training first.
If he has learnt that their crate is an unhappy place to be they might need to move it to a different area, ideally somewhere that they spend a lot of time and leave the door open all the time.
Make all the good things in his life happen in the crate, meals, treats, toys, stuffed kongs, juicy bones, extra fuss. Perhaps part of his "working" could be finding single treats hidden in the room (not too hidden, he doesn't need encouraging to chew through cushions to find treats) but all the best treats will be found in his crate. If he understands a down stay they can train him to do it in the crate and occassionally drop treats in.
They mustn't close the crate door until he willingly goes in to lay down by his own free will. When they do close it it should only be for a minute with them stood/sat right by it. They shouldn't make a big deal of him being let out, if they praise him he could see it as better to be out than in. They should gradually increase the time he is left in the crate and builid up to being able to leave the room for 10 seconds, 30 seconds, a minute, 5 minutes, and so on.
It might be worth them finding a good, kind trainer who is experienced in dealing with this kind of thing to give them some real support.
Karen