Such a tough thing, to be dealing with this not just for yourself but for your little guy as well.

I think it's a fairly common reaction, his world has been upended and it will take a while for him to adjust & feel secure again. Give him constancy, in your words & your actions. Be consistent, do what you say you will, be where you say you will, when you say you will. Tell him the same things over & over, when he says are you going to leave me - 'no, I'm always going to be here' ; 'I guess I'll pack my things and find a new home' - 'well I'd be very sad if you did that because I love you and I love living with you' - I'm sure you already are saying this sort of thing - over and over again. It's what he needs to hear right now.

I second the idea of speaking to a counsellor, if not him, then for you yourself, they might have some good ideas on how to help.

Best of luck.