How do you know what he's dreaming? If he's telling you that, the only purpose is to try and make you jealous. Which would be stupid to be jealous of a dream ... unless he starts acting on it, and trying to cheat on you.
Get him back by dreaming about cheating on him! No, just kidding about that. :) Dreams & meanings of dreams, are subject to much speculation. Keep it in perspective. While it could be indicative of a conscious or subconscious desire to explore other relationships (I suspect that is what you are concerned about, no?) What dreams have you experienced that were not relevant to you, in waking life? Did they disturb you? How? The biggest thing is, your BF is sharing this dream with you. That could mean a couple of different things...like maybe he's prepping you for the inevitable, or, the thing I feel best about...he trusts you, and trusts that you know him better than that. He's told you only because it bothered him. If that's the case, share with him some of the dreams you've had that were, to say the least, disturbing. Let him know he's not alone in dreamland. Best to you both.