What appetite stimulants do you swear by for cats. My vet has prescribed an antihistamine for anorexics.?

My Simone is 8 months old and weighs less than 4 pounds. Her brother is 8 1/2 pounds and huge and athletic. She tests normally, but is not growing. She just had stopped eating entirely and got so sick. The stimulant is working, but she can't stay on it forever. She only will eat ProPlan by Purina (hard food), although I have bought literally every food possible. Any suggestions will be a lifesaver and are thankfully requested.