Heres a possible scenario:
Having a dream about something in particular could mean that you were thinking about it shortly before you fell asleep. Thoughts move to our subconscious shortly after we consciously think about them and when we dream, our mind pulls from our subconscious resulting in a dream about the thoughts we consciously have before we enter into sleep.
For instance, I had a dream about my ex boyfriend Frank. It was right before falling asleep that I wrote a personal message to someone on this forum about my relationship with Frank and how he dumped me and married someone else. I was describing how depressed I became after the breakup and how I did not believe I could meet and fall in love with anyone else. (And as you know, a little over a year later I did meet and fall in love with the man who I have had a 2 year relationship with now.) I no longer desire Frank even though he divorced and started communicating with me again because I moved on to be with the man I am with now. But why would I have a dream about Frank? I believe it was because I had a thought about him while writing about what happened with our relationship. Nothing more, just a conscious thought that moved to my subconscious mind.
The dream you had could also have been prophetic or symbolic (like Lise mentioned.) But more than likely your underlying fears were acting themselves out in your dream. I guess if it were prophetic or symbolic it could be a sign telling you to be prepared with whatever may happen. In any event, I think that when we want something so badly, that even though we try to remain positive about getting it and letting go of the control, that we try to find in everything a sign (or symbol) pointing us in the right direction, or a sign (prophesy) in what to expect for the future. In any event, I believe that you may not have gotten to the point where you have fully let go because you are now plagued with why you would be having a dream about your ex telling you that he had a potentially serious relationship with someone else.
I'm not saying that letting go means we have to forget all about the person and move on to another relationship, I'm just saying that if you have fully let go you will love him unconditionally no matter if he is with someone else, and it should not bother you if he has found someone else. Right now I think you are concerned with what the dream means because you worry if it is telling you that he has found someone else or that he will soon be with someone else. Since it was just a dream, I would tell myself that it was because I was thinking about how much I desire him, that the underlying and subconscious fear of him finding someone else is the reason for the dream. Be grateful that it was only a dream and take it as a sign that you are still in the letting go stage. Remember that your worrying about the dream becoming real may manifest itself as such so that is a good reason to stop being concerned with the dream and go back to feeling good and positive about YOU. Right now you have to be happy with just YOU and no one else. Once you can find the happiness within yourself, everything else will start falling into place.
And remember that we should always think about what we do want instead of what we don't want.