my heart goes out to you as someone once in the same position.
Mine did not phone for a week, mind good job as I would have bloody killed him if I could have got my hands on him, and he knew it, so he went into hiding.
The most important thing here is your son, although I am sure you don't need telling that, try to keep things as "normal" as you possibly can for his sake, don't let on what has happened yet, and if the spineless git does turn up, get a friend round quick to collect your child, so he does not see /hear the pain you are in. My son was 5 when it happened to me, he was a rock and a great comfort and kept me going when I was really low (wish I had found you wonderful ladies on MN then!)
Lots of cuddles are in order.
Like some others have said if she was in your house, this has been planned, and it will not be the first time he has met her, so sorry.
Instead of changing the locks, beyond my capability, get some of those door chains, they are not illegal, and if you are in he can't just then walk in at his will.
The only thing you need to do today is sort out your bank account if you have a shared one, make sure you get enough cash to cover you, so he can't go and empty it out, same for savings, if shared, get your half out of it, as who knows what may happen. Mine was not that evil, but one of my friends OH were, and she had 2 kids, and as well as everything else had no money at all, not even for a pint of milk.
Take each day as it comes, you will get through this, be strong, and don't let him try to twist it, that its somehow your fault, there is no excuse at all, its disgusting, in your house with your child there, I am getting angry for you.
Take support from where ever you can, don't be afraid to ask.
You have handled this with far more dignity then me, so well done.
Big hugs