I am sorry to hear you are worried about Roxy. My name isXXXXX am a veterinarian, and will do my best to help.
The most common reason for a dog to "scoot" or rub his back end on carpet or flooring, are impacted, infected, or inflamed anal glands.
I would have his anal glands checked ( they are inside the anus, and can't be seen) by a groomer or veterinarian.
Eating grass and leaves is common in dogs. We are not sure why they like to eat grass, and it can make them vomit. It does not seem to indicate anything lacking in their diets. They just seem to like it! When I lived where we had grass (none here in the desert) every spring my dogs (and cats!) would look like cows, eating the grass.
Having impacted or inflamed anal glands can be caused by a food allergy. If he is allergic to grass or the leaves, there could be a connection. I have had some dogs with recurrent anal gland problems get better with a diet change.
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