It sounds like your boy is very smart and has already trained you! He knows that you will eventually rock him to sleep -- and what child wouldn't like that? I don't think this is a temporary stage: I think your son will continue his behavior (to get exactly what he wants) until he learns that you will no longer be manipulated. I would recommend finding a sleep training method and sticking to it. There are many books available. With my kids, I have used the sleep training methods of Dr. Marc Weissbluth, "Healthy Sleep Habits, Happy Child," which would urge you to teach him to soothe himself to sleep instead of depending upon you to rock him. This can be done using "cry-it-out" (just let him cry without ever going into the room) or "check-and-console" (let him cry, but enter his room in a specific fashion at increasing longer intervals). Dr. Weissbluth's book can teach you these options, if you choose.
Whatever book or sleep method you choose, your son will NOT be happy if you stop rocking him to sleep -- so be prepared for his behavior to escalate before he comes around to the new behavior you are trying to teach!