I must say, the "leave it" command takes time. And effort! When we got Annie I think I said, "leave it" more than I said any other word.

It seemed like everything was,
leave it. Because as a puppy they need to touch and taste everything! Everything! Just like a human baby. They're young and curious.
Maybe you can practice it as a training session? Not just on a walk or when you suddenly catch him with something in his mouth. But putting something beside him and saying,
leave it. Take it away and reward him with a treat. I've never even done that. Annie just learned it because I repeated it so many freaking times!! lol
so many!
And that instinct to taste everything does fade as they get older. At least for most dogs. Unless it's ingrained and not corrected. Then I think it can really be a problem. I'll never forget watching the dog whisper about the dog that ate stones! It was really an issue! That dog continually wanted to eat stones and it really took a lot, even with Cesar Millan, to correct it. The worst thing is letting it go for too long. Just like any habit. If you catch it and nip it in the bud, you have a much better chance.
And I agree about the muzzle. Maybe that would be a good option when you can't be watching him.
Puppy stage is just like having a crawling baby. They don't get the free roam of the house. They're gated off, everything is baby proofed and watched like a hawk. Maybe keeping a short tab of leash on him to quickly grab him would help.
I think the first year of puppyhood you need four hands and eyes in back of your head. Sometimes on the side of your head too. It's trying! Annie ate her share of things too. Not clothing or socks but just stuff outside. Oh and also the panty liners that I used when she was in heat!

I was beside myself with that! I said, she's going to die!

I didn't know what to do with that. But I finally figured out what works best. I don't use the liners! The panties that I use on her now work for me and her. I had to adjust. Or I was going to lose my mind!
And you remember the corn cob for Fiasco. Dear God. But I can tell she has gotten over a lot of those things. I adjusted a few things and she grew out of a few things.
But it was a constant battle. A constant education. Constant eagle-eyed watching her and teaching her to leave it. It's work!
I hope you guys can come up with something that works for you too. Hang in there!