I'm sorry for your pain. It's not hard to see how much you're hurting, and I am truly sorry.
I hope that in time you will come to see that the experience you've described is not polyamory at all. Polyamory is based in love, concern and respect for all the people concerned in the relationships involved, and if the situation is what you've described, clearly your ex has not demonstrated much respect, love or concern for you and your son.
I don't blame you for lashing out. Hurting people lash out sometimes. Some time back, Fidelio hurt my feelings pretty badly, to the extent that I walked out. During that time, I said some pretty harsh crap about men in general, although only one man in particular had hurt me. I think the kindest thing I said was, "Not all men are pigs; some of them are gay, and some of them are dead." So I know where you're coming from, BrotherMan. In my case, some hot cocoa and a long cooling-off period helped some, but the roses delivered to my door the next morning with a sincere apology really took the sting out of the injury. I'm guessing there are no roses in the offing for you, and not much possibility for the kiss-and-make-up. (Not that your ex deserves another chance anyway.) The situation stinks, and my heart breaks for you.
Be strong, BrotherMan. This too shall pass. I hope that the future holds brighter days for you.
PS: Before any of you poly-peeps start lining up jump my case in defense of the menfolk, let me just say, I LOVE men. I'm a big fan. Some of my best friends sport the Y-chromosome. Seriously. Big fan.