Ex still has my house key and other thingsWatch
I even told him he could drop them in my mailbox or doorstep
TLR; Why isn’t he bringing my stuff to me. Break up consisted off no arguing or anything bad.
Then go to the police about getting your belongings back, i think they can send an escort with you to go get them or it would be a good start for legal proceedings to obtain them.
Edit: Even if you get the key back there's no guarantee he won't have made copies. People can do bad things when angry or if he gets drunk etc
You can risk him using the keys to obtain access to your home whenever he feels like it.
Depending on the nature of the relationship it might be a tad alarmist to summons an emergency locksmith and worry about being held hostage. Could just be a matter of giving him a bit of time to return the key or changing the lock yourself if he is delaying on this