Eating non-food items is known as pica. Your first step is to have your cat checked by the vet to make sure he's not experiencing any medical problems that would cause this behavior. Once your cat has a clean bill of health, there are several steps you can take to address the problem.
First, do a bit of preventative cleaning. Paper should be out of the cat's reach to avoid reinforcing the behavior. However, leave out a couple of pieces of paper that have been treated with Bitter Apple cream. This is a bitter anti-chew product available at pet supply stores. This will help your cat to associate paper-eating with an unpleasant experience.
Finally, try to increase his activity. Siamese cats are athletic and need daily exercise. Use a fishing pole toy and conduct a couple of interactive play sessions every day to avoid boredom.
