I was fortunate w/my CRF girl to never need an appetite stimulant so I have no first hand experience w/cypro.
Does Sampson seem nauseous at all? licking lips, smelling the food like he wants to eat but just walks away, hovering over the food or water dishes? If he is he may need an acid blocker like Zantac ~ to be prescribed & dosed by the vet of course.
There are other food options listed on the table here, this becomes a balancing act between phos content & what kinds of food you want to feed your cat - not all are grain free, or by- product free etc, sometimes it's just a matter of feeding whatever they will eat. http://www.felinecrf.org/canned_food...od_data_tables
You may have already seen this page but here are some more tips to persuade your cat to eat http://www.felinecrf.org