Since internal (gastrointestinal) parasites are shed in the feces intermittently your cat may still have some undetectable GI parasites. Sometimes multiple fecal tests must be performed before finding parasites. Additionally, tape-worms are notoriously difficult to find in a fecal exam. Therefore, it may be a good idea to ask your vet if a deworming with Drontal Plus for cats would be a good idea. Drontal Plus or equivalent is available by prescription from your vet.
There are a number of congenital disorders of various organ systems that can contribute or cause failure to thrive, or failure to gain weight. Please call your cat's breeder and ask if there are any problems in his blood line. Hopefully, the breeder will answer.
You might also ask your vet for a canned Maximum calorie, high protein prescription diet to use as a therapeutic trial. Other helpful supplements: Colostrum and Probiotics.
Incidentally, oranges are not very palatable to cats. You could try your home-made chicken recipe without the oranges and see if he likes it more. A Vitamin C supplement can be used instead or the oranges in your recipe (if that is why you add oranges). One oranges contains about 30 to 50 mg's of Vitamin C.
