Does Brindle like harder food? When I had to give Jessie ID, I would cook some of it up as treats by cutting a can of food into small pieces and cooking the pieces in the oven at 350 for about 20-25 minutes. Sometimes I'd sprinkle it with basil. The vet said it worked for dogs that didn't like wet food, or wanted a little variety in substance.
Of course, that was the canine ID, not feline...but maybe it might help?
I hope his appetite and health improve quickly!