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

I am currently working in IT startup company based in Noida. I joined this company in October 2017 as a trainee. At the time of joining they provided me "Letter of Intent" in which they mentioned that
1) the probation period will be of 6 months and Formal appointment letter will be issued after 6 months.
2) if resigned during probation period then I have to serve notice period of 1 month
Now I have completed more than one year in this company and I haven't received any appointment letter till date. 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 asking me to serve notice period of 60 days. He told me that "you have automatically become permanent employee of his company and as you have completed the probation period, hence your notice period is extended to 3 months."
He told me that if I left the company without serving the notice period then he will not provide any kind of relieving letter and experience letter from his side and he will give negative feedback to my next company if they inquire about me. What should I do ?

Dear friend,
The bosses of some of the start ups, 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 scanty regard for good professional standards or 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 prepared for a show down.
Even according to the so-called "letter of intent" issued at the time of joining the start up, the probation is for a period of six months comprising of a notice period of one month only and you would be issued a formal appointment orders after the said six months. However, as per your statement, even after completion of more than one year service in the same organization, no appointment orders was issued. It implies that still you are a probationer in the absence of a formal declaration of your probation and as such it is suffice 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 which you would be simply walking away.