How do I keep my dog from chasing our cats?

We recently adopted a 9 month old Boston Terrier. He is living up to his terrier name. He will constantly chase and terrorize our two cats. Any suggestions on how to keep him from doing this? We are planning to take him to cles in a few weeks but before that he is driving us (and the cats) crazy. On the one hand we think that he might be "playing" but the cats rarely come around anymore because they are afraid of him. He has never actually bit or hurt them but he does make it difficult for them to move around the house freely.