My suggestions:
1. Kennel train.
- when you can't keep an eye on him, he stays in the kennel to prevent him from acquiring bad habits.
2. Teach attention.
- this is a command you give that will signal the dog to stop what he's doing and look at you at attention. it's like the "heel" command when walking. I use "nuh uh" as my command. This signals my dog to immediately stop what he's doing and look at me. Very useful in many situations - e.g. he's off leash and runs to the road - "nuh uh" makes him stop and look at you which gives you a chance to give him another command to come to you, preventing him from getting run over.
- for this chewing or any undesired behavior, you can immediately give the command which should make him stop what he's doing and look at you which will then give you a chance to give him another command to distract him from the undesired behavior - like telling him to fetch a toy.
- you can teach the attention command in the same way you teach him everything else - through positive praise/reward when he does it right, and repeat and repeat and repeat over and over and over until it becomes habit.
Good luck!