Sick and not eating

I adopted a young cat (about 1) year, a month ago. He just started getting comfortable in the house, and then a few days ago he started having diarrhea. Then he stopped eating and drinking. He's been acting really out of it and losing weight rapidly. So last night I took him to an emergency clinic in the middle of the night. They checked him out and said his kidneys seemed abnormal. But then they did thorough blood tests and found nothing wrong with him aside from a high white blood cell count. The dr gave him a shot of fluids and antibiotics, I took him home and he started acting normal again, but didn't want to eat or drink still. So about 24 hours later, I gave him food and water through a syringe. Then a couple hours after I gave him his antibiotic, he vomited all the food I had fed him. I'm really worried that he is not eating and not able to keep food down. What should I do?

It's really important that you keep trying to feed him, even just syringing little bits at a time. What about trying an appetite stimulant like Forti Flora? It's expensive, but very effective in my experience.
Hopefully the antibiotics can kick this infection and he regains his appetite.
Did the vet not have any idea what the infection could be? That rapid weight loss would worry me.

Thanks for the reply. I just fed him another couple syringes of food.

The vet gave him a prescription food, which is what I've been feeding him in the syringe. I've never heard of Forti Flora though. How does it work? Is it a medicine or a food? Do most pet stores carry it?

The vet seemed to think there was something wrong with his kidneys, but the bloodwork showed them as being healthy. I guess it could be a kidney infection?

I would talk to your regular vet and see what they recommend. You need to find out what's causing the problem.  Also, if he got IV fluids, his appetite will most likely be suppressed.  Try warming up the food a bit and offering it to him.  Add in a touch of water to make it the consistency of oatmeal.  If he doesn't eat it on his own, then you can syringe feed him.  How is his elimination?  Is he still peeing and pooping normally?

I bought Forti Flora from my vet, but I think some people buy it online.
Warming up the food is a good idea too as it smells more appetizing and smell is very important to cats.

He has been peeing normally, but hasn't pooped in days. He only vomited once. The vet called to follow up and said the antibiotics usually take about 48 hours to kick in, so in the meantime I'm keeping a close eye and feeding through the syringe.

It is very common for kitties with kidney problems to also have constipation problems.  Something else you can try to help with the constipation is adding plain canned pumpkin to his wet food or adding Slippery Elm Bark.  The SEB helps soothe the intestinal tract and soften the stool.