All of your memories of your ex-boyfriend are still contained within the organic matter of your brain. While dreaming, your brain constructs worlds all created from those memories. These worlds will contain people you've met, places you've been, and things you've experienced. All of these things combine into new things, but are originally composed of everything in your head. Some people, places, things have a stronger presence in your memory than others like the house where you grew up, your first pet, and probably boys that you have had feelings for. It's natural for these things to pop into your dreams. In a year you will have met new people and they might be in your dreams someday.
I think the question of whether or not you still have feelings for him could be answered with different supporting data.