Oh no, so sorry about Diablo's health issues! Do you know what sort of blood work was done and can you get copies of all the results? What about other tests, like a urinalysis?
What is his stool like? (ie soft? hard? colour?)
What does he eat?
Is he an indoor cat or does he have access to the outdoors? Feline leukemia and FIV are retroviruses that are communicable between cats, either from a bite, passed down from the mother cat, or from exchange of bodily fluids.
If he tests negative for FeLV and FIV, would a specialist or an ultrasound be an option? Perhaps a holistic vet could also give you more insight.
In the meantime, it's crucial that he eat. Try offering him anything you can think of that might help jump-start his appetite. Plain chicken baby food, pieces of cheese, boiled chicken breast or some KFC with the skin removed, canned salmon, sardines, Temptations cat treats, stinky canned Fancy Feast with nutritional yeast sprinkled on top.... whatever it takes. Here are some more tips:
I hope you get to the bottom of this soon!