Ah, that is very annoying. Have you tried crate training? Matilda hasn't chewed anything that wasn't hers in my home, because she is being crate trained and therefore doesn't have the opportunity! For almost a month, she was under 100% supervision every time she was out of her crate, so it was very easy to correct her if she started chewing on the wrong things. You can even use a leash in the home to make sure he doesn't get away from you. After she stopped trying to chew, she got a little more freedom. Now she mostly stays out while we're home (unless I'm working from home) and is only in the crate at night/when we're gone.
As for "drop it", Matilda's trainer taught me a great trick! Keep treats out and handy since this seems to be a constant thing. When Otis grabs something that isn't his, say "drop it" and shove the treat in front of his face. You basically trade him. He drops it for the treat, and gets the reward. He'll learn soon enough what is his to chew and what isn't. Dogs want to please you, you just have to explain to them (in a way they understand) how to do so. If you're not having aggression otherwise (especially food aggression), it seems like this will be very easy to fix!
And you could always consult a professional - just to teach him basic manners!