It's probably one of two things. It's either stuck to the fur on the outside of his body, or it's stuck to the fur on the inside of his body.
For the first problem, some people shave their cat's but area. For the second problem, you might want to look into some hairball treatment and groom him more often to help reduce the amount of fur going into his gut.
The content on cat-lovers-only.com is for education and information only. It should not be used as a substitute for professional veterinary advice, diagnosis, or treatment. For medical advice about your cat, please see your veterinarian.
As an Amazon Associate I earn from qualifying purchases.