Please help Jax

My poor cat Jax got sick out of nowhere. He is only 5 and the sweetest cat but he became lethargic and I thought he had a reaction to flea meds. I had to rush him to the er vet and now he will be there for at least 2 days. His wbc is super low and low blood sugar. It could possibly be a really bad infection but they have to run a bunch of tests just to find out what is wrong before he can be treated. I just lost my 18 yr old cat a year ago and this is throwing me in a bad place. I can't afford the bills all over again as I am a stay at home mom with leukemia and bills are hard as it is. I don't want to lose Jax too. If anyone can help at all I would appreciate it.