Thanks for the additional information.
This is very suggestive of pica. Pica is a disease process where for some or other reason the minerals inside the body such as phosphorus and calcium and iron are not balanced. This presents with all the signs that you are seeing.
It is most unusual for dogs on good diets to show these signs. As the diets are often well balanced. I do not know the food brand but have Googled it. It looks all in order except that I cannot see a specific giant breed made diet, perhaps there is, you would be the best to see this.
There are two things to consider:
Either he is not getting enough of the minerals in the diet (phosphorus, calcium and iron)
or he does get enough but his body cannot make use of them.
If you trust that the diet is well balanced and that Grizzly is getting all he needs than you would need to consider a problem inside Grizzly that does not allow him to make use of these minerals.
Diseases such as Inflammatory bowl disease, exocrine pancreatic insufficiency, kidney disorders, certain cancers, diabetes mellitus and other endocrine disorders can all
be the cause of this. It is very difficult to narrow down the actual cause without running some blood tests.
The diet needs to be ruled out. You can do this by changing his diet to a veterinary based diet on a trial basis. I would say 2 months is a good time frame. He is a giant breed dog, so the chance of him not getting enough phosphorus and calcium in the current diet is likely and should be ruled out.
If the diet change does not work than you will need to visit the vet to run diagnostics to check for a medical cause of this behavior. IBD and Exocrine pancreatic insufficiency tend to be the most common causes.
If all the blood tests come back than Pica due to behavior reasons is diagnosed on exclusion. This will require a lot of work to try stop this behavior. Some medications can help to make this task easier. I would work with a vet and behaviorist to sort this out. Luckily this is very rear.
So the diet should be tested, than to rule out medical causes and we hope that it is not behavioral.
I hope that this answer helps you find the cause of pica in Grizzly (awesome name for a foundy).