Cat Prescribed new diet but won’t stop throwing it up.

Hello, firstly, my Vet isn’t open until the 8th because of the holiday weekend, so I do plan on taking him in once they’re open.

Last weekend my cat was having trouble peeing, he kept going in and out of the litter box but nothing was coming out. I called our vet and they told us that is COULD be an Emergancy and we should take him into an ER Vet. So we did. The vet wasn’t able to see him for a long time so he was pretty pissed, they said it was difficult to examine him because he was so mad, but that it was probably Feline Lower Urinary Tract Disease, and prescribed us a food.

I asked if we should slowly transition him to the food and the vet suggested that we switch him over immediately instead of slowly. She said something along the lines of “A little bit of diarrhea is not a big deal, I’d rather he get on the food sooner”

So we bought the Hills Science Diet Prescription food. He immediately threw it up. I thought at first he was just so hungry or he liked the food so much that he was eating it too fast. (He was throwing up whole pieces) So we tried slow feeders. Even if he only eats a small amount he throws it up, he’s been able to keep a little down but not much.

We tried the wet food as well, giving him a very small amount at first but after a few minutes he throws it up.

I noticed today when we tried the wet food again that he shakes his tail and head strangely after eating it and now I’m wondering if he’s allergic to it.

I’m getting him into the vet ASAP, but I’m at a loss right now. Should I give him some of his old food so he gets something in his system?

Any advice is appreciated.