OP, first of all, I hope you are ok. This was a difficult thing to do, but if it's right for you and your child and your home, it was the right decision. That's your first step in the right direction.
It is quite telling that, from what you write, his first thoughts are for himself. As they would be, and as they would be for you, if your situation had been reversed. But, it should give you a good indication of where you stand in his order of priorities.
While he is packing, are you certain he is not taking anything of yours, of value, or any important paperwork that you might need? Just check.
Next steps while he is in shock, before he gets to the denial and then bargaining stages:
As others have said, it's probably best that he goes somewhere else "to think " or you will find him still in your house when he starts denying and bargaining. You are not throwing him out on the street, he's (apparently) a grownup, so he can go to a hotel or a friend's house, as I assume his children are with the ex?
Once he is gone, you can sit, have a cup of coffee or tea (no alcohol, you need a clear head), get the tub of ice cream out of the freezer and eat as much of it as you want. Give yourself half an hour, then call a locksmith, if it isn't too late today (or do it first thing tomorrow morning). Change all the locks, front door, back door, side gate, make sure the garage door is barred from inside, and your car and keys are secured.
Once you are secure inside your own boundaries again, you can cry or collapse as much as you need to, given that you have a child to care for. All your crappy emotions can now happen within a safe space. If you want to discuss things with him another day, you can do this now that your boundaries are firm.
I'm sorry you've had to do this, but you will be stronger for it. It will take time, so be gentle with yourself.