Hi there! I'm so sorry that you've having a difficult time with Sabastian. It's very frustrating (both for you AND your vet) when it's clear that something is amiss but it doesn't show up on blood work.
When this happens with my patients, I usually give a thorough broad spectrum deworming (just in case, because fecal exams do not always show intestinal parasites). If the kidneys check out OK on lab work, I also discuss trying kitten food because of its higher calorie count. Alternatively, you could try a prescription diet like a/d - this is very high in easily metabolizable energy and is very palatable to cats (it's YUMMY!).
Pay special attention to his eating habits looking for signs that might indicate oral pain like reluctance to chew hard food, dropping food, excessive drooling, or pawing or rubbing the face.
When I have a sick patient with normal blood work I usually try the above things and recheck their exam and weight in about a month. If they are still losing weight, I move on to further diagnostics like survey radiographs (X-rays of the whole body) and possibly an abdominal ultrasound.
Unfortunately, sometimes when lab work is coming up blank, it can be indicative of cancer somewhere in the body. There's no one simple test for cancer, so it often shows as a sick pet that we are having a difficult time diagnosing.
Please keep in mind that I'm making an educated guess based on your description and my education and experience. Ultimately the best person to advise you will be the vet who has examined Sabastian most recently.
Please let me know if I can answer any other specific questions!