Notice period extended without prior notice - "you have automatically become permanent employee"

I am currently working in an IT startup company based in Noida. I joined this company in October 2017 as a trainee. At the time of joining, they provided me with a "Letter of Intent" in which they mentioned that:

1) The probation period will be 6 months, and a formal appointment letter will be issued after 6 months.

2) If I resigned during the probation period, then I have to serve a notice period of 1 month.

Now, I have completed more than one year in this company, and I haven't received any appointment letter to date. A few days back, I forwarded my resignation letter to my boss (There is no HR or manager). After that, we had a verbal communication, and he is asking me to serve a notice period of 60 days. He told me that "you have automatically become a permanent employee of this company, and as you have completed the probation period, hence your notice period is extended to 3 months."

He also mentioned that if I left the company without serving the notice period, he would not provide any kind of relieving letter and experience letter from his side. Furthermore, he threatened to give negative feedback to my next company if they inquire about me. What should I do?

Dear friend,

The bosses of some of the startups, in general, try to live up to the meme "Rule 1: The boss is always right. Rule 2: If the boss is wrong, see Rule 1." They are their own Prime Ministers and as such, they have scant regard for good professional standards or the legality of their actions. Therefore, in the current scenario you described, it is for you to decide whether you want a smooth exit in view of a peaceful future employment or are prepared for a showdown.

Even according to the so-called "letter of intent" issued at the time of joining the startup, the probation is for a period of six months comprising a notice period of one month only, and you would be issued formal appointment orders after the said six months. However, as per your statement, even after completing more than one year of service in the same organization, no appointment orders were issued. It implies that you are still a probationer in the absence of a formal declaration of your probation, and as such, it is sufficient to give one month's notice on your resignation.

So, write this back to him politely and request to relieve you on the expiry of the notice period of one month. Failing this, you would simply walk away.