He may be curious as to what you are doing now and just being nosey, possibly misses you, or being obsessive.
Call the police
yes.
no
"He is your ex boyfriend" is correct.If you were to say "he was your ex boyfriend" it would suggest that he is no longer your ex.
depends did you break up with him? call him
maybe he wants you back.
call up a hitman
i mean, if he's your ex-boyfriend he's your EX-boyfriend. that means...boyfriend.
because he is thinking about her or still in love with her
If your ex boyfriend is in your house without your permission, you need to immediately get help. Call the police, or if he has the key to your house, get the lock changed.
If he come to the house you are living in and is abusive then you can ask him to leave. If he refuses to leave then call Local Police and explain why you are calling. The Police should then come to the house and talk with both you and the ex-boyfriend. Police will then decide if ex-boyfriend should be arrested or not.
If you mean by you dated someone, then broke up it's ex-girlfriend or ex-boyfriend or just ex. If you mean you dated someone, broke up, and then started dating again its girlfriend or boyfriend.