You could try to buy some salt free beef or chicken broth and adding some to water and mixing some water into his food so he is getting enough water. If you measure the water before putting it in the cat bowl then measure the water later in the day you'll get an idea how much water he is drinking . If he drinking a lot of water and not peeing you need to call the vet and if he drinking no water he'll need to have an IV . I am sorry to hear your cat is ill. You may want to check with your vet to see if it OK to give your cat the broth .
I had to put broth in my dog water when he was not drinking enough water.