you tell your son the truth, that might sound hard but lying to him won't do you or him any good. And cheer up. losing your girlfriend isn't the worst thing that could happen to you be happy that you're alive and healthy and that you have a son!
And yes, you actually do have the right to be pissed off. i'd be really pissed off, especially if she throws it all away like that!
anyways, try to look at the bright side, not the bad side. the bad side will only tear you down, and believe me that you don't want to go there