@RudaRudoRude
I'm so sorry you're dealing with this!
First off, remember that you are strong. You are!!! You have made the first strong step already; telling him not to return.
Since it sounds as if he (abetted by his mother) is not going to 'go quietly' you will need to continue to be strong and take some needed steps.
First, yes, log a harassment complaint with the police since they've said you can do so. This is important. You need to begin building a 'paper record' of his actions. If he knew that your wonderful FM was with you, stress this to the police. I see you've done this. Good for you!!!
Second (and yes, this is controversial) if you feel there is the least chance that he will try to gain entry, change your locks or the lock barrels. Let your landlord know (and why) and give them the new key. Keep the old lock and key in case the LL wants it put back if/when you move. The home is in your sole name and was your pre-marital residence. The worst that could happen is that you plead ignorance based on those facts and you'll have to give him a key. If that's too big a step for you, get chains installed on the doors. It won't stop him from entering when you're gone, but will prevent a 'nasty surprise'.
Third, if there are any joint finances, separate them. Take exactly half, open accounts in your name and put it there.
Fourth, see a solicitor. Lay everything out before them and ask what actions you need to take. Take FM or FD (foster dad) with you as a 2nd set of ears.
I know, this is a lot. It doesn't all have to be done tomorrow. Just start taking steps to get it done as you are able.
ADDED; I see you're going to FM's. Good. If you have important papers at your place (birth certs, marriage cert, passports, financial records) take them with you and store them at FM's). Also, changing locks won't be that urgent if you aren't going to be there, other than to safeguard papers and possessions.