Subleasing a part of the house is usually based on trust. So talk to him and try to convince him that it is not possible for you to pay the rent anymore.

If he still continues to nag you, turn the table around.

Tell him you are going to call the cops and complain that he has been constantly calling you up and harassing you. Simple.

As long as you do not have any documents tying you to the apartment, you are safe.

Try the nice guy route first and if nothing works out, Play D BAD GUY !!!

All the best. Cheerios