You stop him stealing from you by stopping being anywhere near him. His stealing is not “out of character”. This is his character and anything else he’s made you believe about him was merely acting in order to get access to his personal ATM, which is how he sees you and whoever he’s conned into being his “best friend”.

I hope you changed your lock after he stole your key. If not you need to do that.

I’m very sorry but there is no good future in this relationship. He’s a thief and a conman.