If it's anxiety, we had great success using a 'toy box'.
(*) A big plastic box filled with screwed up bits of newspaper, cardboard, some of his favourite toys, the occasional little treat. Get him used to being given his Toy Box, using the phrase "Toy Box"with appropriate excitement, so that he loves it and excitedly dives in to find the goodies therein. Then gradually slip out of the room while he's in there, not for long at all at first and then for increasing amounts of time. If he doesn't notice you leave, the anxiety is much less and may even be absent.
We used this with a rescue GSD x Collie who had severe separation anxiety. It took some weeks to build it up so we could go out of the house, but it did work. After not long we didn't need the Toy Box at all, he just accepted that we left and came back.
For the rest of his life, though, if he did get anxious, he'd find some newspaper to chew up. And if that wasn't enough, find a plastic container full of things to discover - namely the kitchen bin
- empty it and spread its contents all over the floor.
I used to leave Radio 4 on for company for him. Occasionally he'd have got anxious and chewed up some newspaper. I eventually worked out that it was John Humphries haranguing someone that upset him. I retuned to Radio 3 and hardly ever had the problem again.
(*) For full details on the 'Toy box' technique, see John Cree's book, 'Your Problem Dog'.