It sounds like he’s just a gross weirdo. Lots of dogs insist on eating their nail clippings or those tufts of undercoat you can pull out when they’re blowing coat, so it could just be a quirk of your dog if he’s done it all his life.
If it’s a new behaviour, it could be triggered by some sort of deficiency. If the behaviour only recently started, I’d bring it up at the next vet appointment he goes to. Otherwise, it’s probably just a dog thing. -C