Sick Acting Strange?

Is he scratching? Does he show any sign of a runny nose or eyes? Any sneezing?

Head tilt is often a sign of an ear infection. Prompt antibiotic treatment may resolve it, and the head tilt may or may not go away. But even if the tilt remains, it shouldn't cause him any problems.

The butt-twitching may be due to mites, although I think you would also see him scratching if that's the case.

The two together, however, could possibly indicate a neurological problem. If he were mine, I'd get him in to a good exotic vet for a check-up. Don't bother with a dog-and-cat vet -- they're not trained in the care of exotics. Find a good exotic vet and get him checked out.