I would love to be able to give you meanigful advice, but the truth is, I can't image what could be wrong.

But, my best suggestion is find a new vet!! There is always something that can be done to begin diagnosis without it costing into the thousands of dollars!

An xray, blood tests, neither should be that costly. (of course, I don't know where you live)

I have not dealt with feline leukemia, and I suppose it could explain his sudden illness, but I would keep on syirnge feeding him and try to get him into a different vet before you give up on him.

I will pray for an answer.