Buff, I am sorry for your pain. I cannot say I have lived what you are going through, but I do understand how isolated and alone you must feel. Especially with a small child to care for.
Obsessing over him will do you no good, but you already know that. I do think that asking questions is a good thing, however. Cry if you need to, vent and rage on here or in a journal if that helps. Journaling helped me a lot when I was in the depths of pain and despair.
So, if you can, and when you are ready, take that energy you are using to try and figure out answers about him and try to find some answers about yourself. Ask yourself why you looked for validation from someone who is truly not worthy of your love. Ask yourself why you care how he ended up with them or if he talked about you. Ask yourself why it matters that he has a boatload of fair weather friends. When you can ask those kinds of questions, and answer them truthfully (to yourself) you will be much closer to ending the pain you feel right now.
I know you wish it could stop hurting right away, but unfortunately, it takes time. And (another cliche, sorry) the only way out is through. Take comfort in knowing you are in the middle of going through and you will get to the other side.
Meanwhile, be extra nice to yourself and your son, okay?
L