Most kitty flus are viral, so yeah, like human flus, there's not much one can do unless a secondary infection pops up that's bacterial. So if his eyes and nose aren't goopy and he's still eating, playing, grooming, peeing and pooping, just keep him comfortable, warm and well-fed.

Adding some lysine to his food might help him get over it faster. Lysine is an amino acid, and one big group of viruses that cause the cat flu (herpesvirus) metabolize arginine (another amino acid). Lysine is an arginine "antagonist", so adding 500 mg a day works quite well because it basically deprives the viruses of their fuel and it doesn't do any harm to the cat. It also works well for human cold sores, which are caused by the same family of viruses. I've used lysine on both myself and my cats and it works like a charm. If you're interested, you can find lysine for fairly cheap at most grocery stores, it's usually in the vitamin section near where all the bodybuilding supplements are.