Be sure that he gets hydration (water), you may need to give him water with a syringe. Becoming dehydrated is serious on its own and can aggravate your cats condition. And if he isn't eating more pretty soon, you should consider force feeding as well.
We have an article in our health pages (lower right of the page) about syringe feeding/ hydration that I recommend you take a look at.
Otherwise, it's difficult to give much advice, as we don't know what your vet said regarding your cats condition. We would be more than glad to try to help, if you would like to post more information.
Is the fever due to an upper respiratory infection or is it something else?