Mirtazapine is an appetite stimulant. I prefer Cyproheptadine, as it has fewer side effects, but it also doesn't work for all cats, and Cinco seems to be one of those.

The side effects of Mirtz are hyperactivity, racing heart and lots of vocalizing. But it usually will get them eating. I don't think Shoes is to that point yet. Other tricks I use (which you may have already tried) include sprinkling Forti Flora on top of the food (some kitties love it, others don't care for it - I have one of each), crushing some freeze-dried treats on the top, warming the food slightly, and adding shredded cheese or Parmesean to the top.
I doubt it was the Friskies that upset Shoes' tummy. Had he had that flavor before?