Not served notice period

Hello Experts, I had worked with the company for almost 5 months. I was on probation period of 3 months. Even after completion of my probation period I was not given any confirmation letter. I was also not given any appointment letter nor any contract which states I have to serve notice period of 30 days. I left my job after working for 5 months and informed my boss on whatsapp. he said ok. Now after 15 days he has sent me termination letter stating that you are no longer company employee and also demands for 30 days salary. he has not paid me 15 days salary to me. Even I have not demanded. Now he is calling me everyday and send me emails for working for company for 7 days. He has terminated me and asking me for 30 days salary, simultaneously he is forcing me to work for company and threatening me for legal notice. My question is what legal action can he take when there is no agreement or any documents stating that I have to serve notice period of 30 days.? please guide me as he is threatening me everyday.