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There are several possibilities that could be causing this behavior.
It is possible he has worms. Tapeworm segments are the size of a grain of rice, and they will crawl around the anus until they dry up. You will also see them on a fresh bowel movement. Look under his tail and see if you see anything that looks like this. Cats get tapeworms from fleas and from eating mice, so if your cat goes outdoors, this is definately a possibility. If you suspect tapeworms, you will need to have your vet give your cat a deworming.
Another possiblity is that he has an impaction in one or both of his anal glands. These glands lie under the skin of the anus, and usually secrete fluid when they have a BM. Sometimes they get clogged, and this causes discomfort. So, the animal will drag it's bottom on the ground in order to try to relieve the problem. You may need your vet to express these glands in order for the cat to become more comfortable.
Finally, if your cat has long hair, sometimes they will "wipe" their butt on the carpet after having a BM. Especially if they get any feces on the hair around the anus and it irritates them. Checking under your cat's tail is the best way to determine if this is simply a hygiene problem.
You may need your vet's help in order to determine the problem, so if this behavior persists, you should make an appointment.
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