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With most parasites, you will not see worms in the stool. You only see eggs in the stool and they can only be seen with a microscope. If you suspect parasites, you should take a fecal sample by your vet for diagnosis and medication.
I suggest that you change his diet to a simple diet of boiled chicken and rice with no bones or skin. Give this at least twice a day for the next 48 hours. You should see the symptoms begin to improve. Don't give any treats or table scraps during this time.
If this does not help, you need to see your vet to rule out a bigger problem.
Let me know if you still have questions.