It can take a few days for the medications you listed to work if the problem is bacterial or a protozoan parasite.
Mix that cooked chicken with 1/3 meat to 2/3 rice. You can also add some plain canned pumpkin or some cooked sweet potato for fiber if you suspect there may be a paper impaction there that needs help passing through.
Did the vet check for impaction with an x-ray? if not you might want to check that if there is no real improvement.
See if you can return unused canned food to the vet for credit on your bill.
Also I'd change the water bowl over to plain pedialyte or a mix of 1 liter water to 2 tablespoons sugar and 1 teaspoon salt to see if that helps him hold down fluids and perks up his appetite.
Give the meds another day or two to see if they will finally kick in and solve the problem.
Hope this helps you!

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