Never tie a dog outside in a harness, many harnesses are easy to come off ! dogs are different in shape, my yorkie has so thick body but little feet and head so it comes of easily.
Is the collar tigh enough ? its enough to fit 2 fingers under the collar, if he can put his jaw under it and gnaw on it then its probably too loose !
Always be with him when he's tied outside , dogs can get stuck in the leash and hurt themselfs badly, I know a dog that choked because he got stuck in the leash, the leash got around his neck somehow and he died sadly
So never leave them alone tied ! its so dangerous.
If you see him chewing on his collar then ban him that ! just say a plain ''NO'' and stop him if he's starting to chew it, I would use a leather collar that is thick, hard to chew on it , and when he doesn't chew on it then say ''good boy !'' for example,with a voice like your talking to a baby, right away when he's stopped chewing then do it too and have some treat for him.
Take the collar also off when he's not around you.
Is he microchipped?