That's good that you took him to a vet.
Btw, don't be afraid to force feed him. You can get a syringe from the vet and fill it up with whatever wet food he's on. Plain yogurt is also good for cats when they're sick and can't handle much. Doing the same for water is ideal too. Cats almost never drink enough water, so I doubt he's drinking much at all while he's sick.
For the future, if your cat hasn't eaten in a day, whether you notice any symptoms or not, take him to the vet. There's probably something up and just from the lack of food alone, sometimes cats get extremely ill.