I apologize that you feel you did not gain anything from our conversation. I provided several diagnostic options that it seems your vet has not yet performed. There is unfortunately only so much we can do via a messaging system, without having all of the diagnostics and tests your vet has already run. I am happy to opt out and let you see if another veterinarian can provide additional input for you.
Given the rapid weight loss & no appetite, the possibilities are broad, but my first concern is intestinal parasites. If he has not had a fecal sample evaluated and has not been given a broad spectrum dewormer, that is where I would start. I would also want to have baseline blood work that includes a FeLV/FIV test.
If he had a virus known as FIP, I would expect his x ray to have shown fluid in his chest and belly, so it is unlikely. Panleukopenia is also possible, it causes kittens to just become sicker and sicker. It can be tested by actually using a dog Parvo virus test.
Typically, when they have ingested something poisonous, one of two things occur: 1. there will be abnormalities on blood work like liver or kidney elevations 2. They will have tummy upset (vomiting/diarrhea/inappetance) for 1-2 days that supportive care will treat.
If the vet has not run the blood, I would ask them to. A full panel and FeLV/FIV test is the next best step for your kitty.