there is a product you can buy from petsmart, called "outdoor no" for cats. My suggestion is not to put it right on the car, but to cover it with a sheet or even just get some cheap plastic you'd use when painting to keep the floors fresh. Spray generously. Keep that on the car for about a week, should have broken him of the habit by then.

Much better for your conscious than harming the poor dear, much better for the car than citrus (so bad for the paint!), and it's a fairly cheap fix. Plus if you sorta like having the little guy around...