Help. My cat is still so sick...

So Aiden is a 7yr old always been healthy boy.
Last fri. I started him and his brother on an allergy med. From vet for heavy sneezing. His brother took to it fine however I noticed Aiden threw up, then again, then I noticed he wasn't eating or drinking come Sunday. Took him to vet we assumed it was a reaction to the meds. So I stopped giving it to him, he gave a cortisone shot for boost of appetite. Still nothing. I tried wet food, his treats, as food. Nothing. No water either. Long story long,it's sat. A week later. He has been to the vet everyday this week. Spent two nights getting fluids. They say when he's there he doesn't get sick but still jo eating or drinking. I just brought him home, he just threw uo, again. He's sluggish, sleepy, and just not the same. I've spent well over a grand sense mon. I just got laid off thurs.(couldn't be at a worse time) all his blood work, kidney, liver perfect. What do I do?? Vet said internal doc. They want $400. Just for ultrasound, $120. To walk in and $150. To go over results. This is with no treatment. I'm so upset, feel helpless.
What should and can I do now??

What have you been feeding him before he took ill ?

Food recommend by vet. Heavy in protein.
They were fine with it.

Just take a look at the ingredients, did the allergies start with the new food ? The protein is good, but make sure

there is no corn, wheat, soy, or gluten.
Is this a food that the vet is selling you ? Hope it isn't Science Diet or Royal Canin.

No. It wasn't. He started getting sick after taking allergie medicine for sneezing. He had no prior problems.

So he was on the food prior to starting the medication?  Did the vet check for blockages?  Did he do an x-ray? 

Is there an animal rescue place in your area, often can help with the funding or recommend someone who can help or possibly set up a payment plan

No. He said Monday he will if he's not better. He said due to licking lips and vomiting he things it's internal probs.
We've been filling him with fluids because he won't eat or drink. Also nausea meds. And an antibiotic for a fever on Tuesday.
Yes, he had been well familiar with the food. The only time this started is after allergie meds. But after fluids that should have flushed anything out.
so I'm clueless at this point. He's laying on my lap, I put him in front of food and water but he won't have it. I even tried a piece of tuna, he licked his lips and walked away....