What does count as "something new" in the field for PhD? I am doing PhD in AI field mainly in NLP
my supervisor told that I have to contribute something new to the field.
OK, I started doing some trails in improving one case in the field of NLP AI
I found a way that can improve that case in 80% of the times.
I don't want to go into the details of the method that I found, as it is mainly technical.
Anyway, I told my supervisor about what I found and he wasn't impressed.
The things he mentioned that this method is not provable, which means it is proved only by trying the cases without mathimatical prove.
is this enough for PhD or shall I prove it mathematically?
Your thesis committee, and your supervisor in particular, are the ones who decide if your research is sufficiently novel.
Make sure they are happy and seek their guidance if they are not. It seems your supervisor isn't happy, so work with them to find what you can add or what other direction you can take. You could provide a StackExchange answer with dozens of upvotes saying your work is a novel contribution and it won't get you closer to graduating.