Oh Lisamarie - my heart goes out to you and your sweet little boy. I'm thinking of my own dh right now - because he could have been your little boy having that conversation with his mom 30 years ago. But she never told him - his dad just wasn't there, (dh was 2 when heis dad committed suicide) and he didn't find out until he got a copy of the death certificate for the financial aid office at college. His whole world just fell apart - can you imagine never knowing and then seeing that in print as a teenager?
I wish he had a mom that had been honest with him as you are, and even cuddled him (thats a seperate rant!

) but his mom dealt with it by well, not dealing with it. Now she is more open about it and talks about taking him to outpatient psychiatric care - but I don't really trust her to be telling the truth now after all this time. (She comes out with this after one of dh's brother's has an "incident" and is hospitalized - now suddenly its OK to talk about?)
I'm not a person that generally believes in such things but I have to tell you a story and you can believe it or not...
Back when dh and I were dating one of my friends hosted a psychic party - she invited a psychic over and a bunch of freinds and we each got 1/2 an hour in privacy with her. I'd never done such a thing and it sounded like fun - but it actually wasn't. Someting about the room or the night - but everybody she talked with she connected on some pain in their lives and did a lot of communicating with spirits - and with all of us she was right on target. As each person came out the room got sadder and sadder - some people didn't even want to talk about their session, one woman for instance had been told about a son she had given up for adoption and it brought up so much sadness for her.
For me - we started out going over some tarot cards, simply enough, then she said there's a man here who really wants to talk to you and I was startled. She said he's the father of someone you love and he really wants to welcome you to the family. He feels like you will help heal the family. She asked me - who is it? Do you know why he died? And I didn't then, but she told me "he's really sorry." "He misses his family so much, and he just didn't see another way." "He knows that his wife is giving you a hard time, but he asks you to be patient with her, things were so hard for her, and you are going to help her and he is so happy that you are coming into the family."
I'm tearing up right now remembering the feeling. It was one of so much peace and love, I had a strong feeling of his presence, of his sorrow at having done what he did, of leaving his family whom he loved so much. I wish that my dh could have had that experience - whatever it was, but I do believe it was real. I guess it made me feel like - they can hear us even when we can't hear them. And that if he's anything like my FIL, he's in a good place now - but he carries his sadness at what he did, and your anger is totally justified, and he accepts that. It will never take away what happened or give you understanding of why he left when he did. And I don't even know if you believe this story from somebody over the internet that you don't even really know or wonder what relation it has to your life, I don't blame you.
I guess I just wanted to let you know that my dh is the most amazing man, a wonderful father and husband and I'm sure your son will be too one day, all the more so for being surrounded with the love that you give him and that he has from his great stepdad. I wish you strength and courage in this - I can see you're already approaching it with a lot of bravery and honesty and I'm so so glad. You are a wonderful wonderful mom - I send tons of e-hugs!