Absentee father trying to make things right?

My son was born when I was 16 years old. I moved about six months after he was born, halfway across the country. My family fell on hard times financially and moved to be closer to family. The original plan was for the two of them to move out there with me and for us to be a big happy family. Well that fell through. Things didn't go according to plan, I couldn't afford to move them and we ultimately broke up. I lost contact and as terrible as it sounds, sort of moved on. However, I think about my son on a daily basis and how I was going to ultimately make things right.

Well, I recently got back in contact with his mother and she is willing to work with me. She is willing to keep the past in the past, as am I. I'm still living 1000 miles away, so my first step is saving up enough money to move back. I have a life now with a fiancé and bills to pay. My fiancé is on board with moving, so it's really just the financial aspect of things. But my question is how do I build trust with my ex and a relationship with my son. He's now four years old and doesn't know who I am. She's agreed to let me talk to him on the phone in a few weeks, since I won't be able to move for awhile. I'm great with kids, I'm currently raising my fiancés niece.

I just want to know how to go about making up for years of lost time. How do I get to know him personally? How do I explain to him where I have been? I want to just jump right in, but I know that I can't do that. So any advice would be appreciated.