contacting post-doc in a lab to join his research instead of his mentor
For this short answer, I'm using information from an earlier question that you asked here; namely, that you are a 2nd year undergraduate student. I'm not familiar with how Asian universities operate, but the below would by and large apply to both Europe and the U.S.
If there is any researcher at your university you are interested in working with (probably as an undergraduate researcher), then just talk to them. If you hit it off both personally and academically, then the postdoc (in the case at hand) will discuss with their PI (professor and research supervisor) if they can take you in. The PI will then decide if yes; sometimes after first talking to you as well.