Ugh. What a horrible little shit he is.
So after the cruel way he's treated you, the nasty, spiteful, hurtful and downright low things he said in the aftermath, not only to your face, but via txt and on social media, he thinks that he can show up, cry 'mistake' and you'll take him back? The bloke has no respect for you at all does he?
He's such an arrogant turd that he thinks you're so weak and so full of love for him that you'd be grateful he'd come back!
OP, you are magnificent. You don't deserve to be treated so poorly and you'll certainly be better without that silly little boy in your life.
As for the flat, speak to your landlord and see whether there's some way of protecting your part of the deposit - perhaps get LL to inspect the flat when you move out so that he'd know the if there were any damage, it wasn't done by you. It may also be worth seeing if there's any legal way of signing over the tenancy to the Dickweasel ex for the duration of the notice period so that you're not liable in any way.
Protect yourself, because it's obvious that this pathetic excuse for a man has no regard for you at all and it's possible that in anger he could try to sabotage you in any way he can, such as causing you to lose money or become liable for damage he'd made.
I really do hope that the move goes well and you'll find peace and happiness in your new life.