Does he have tremors while he's awake as well? As long as he appears to be normal while he's awake, he's not experiencing tremors, he's simply dreaming. My cats twitch in their sleep, they paddle their feet, their whiskers twitch and sometimes they even chatter or make these squeaky little meowing sounds while they're sleeping. Chasing some mouse or bird through a lush forest would be my guess. :)
Ghilly
Our Waldo does this too. I was worried at first, but the response seemed to be the same here. He only seems to do it when he's laying down and relaxed.
He must be dreaming of you rubbing his belly :)
Twitching is normal when cats, or dog sleep. Probably any animal out there twitches. Looks funny, doesn't it?