If you need to move anyway, take this as an opportunity for a big declutter and only take the things with you that you want in your new home, new life.
Get rid of the excess, useless, worn out crap that drags you down, costs you money to store and brings no joy to you.
Discard it, dump it and walk away with your lovely kid.
Start your new stage afresh.
As in - don't give him a key. Don't give him your new address until you need to for divorce proceedings. Communicate only via formal emails (and keep them).
Can we help you compose an email setting out what you're going to do and how things are going to be in future?
I'd be worried about his mum, though. Might be worth a message to social services because it may be a case of elder abuse / financial abuse going on.
And do get one of those free solicitor consultations and find out about divorce proceedings. You deserve better than this.
I wonder if him walking out back to his mum's could be taken as him abandoning you?