How to deal with my academic advisor who offered me a postdoc position and try to prevent me getting other offers? I recently graduated and my co-chair offered me a postdoc position which I haven't started yet. I first thought he was trying to help me. Now I am getting other good offers from others. 

I received a very good postdoc offer and was in a very good lab. I told my co-chair, that I will leave if I get that one. After the interview, the main PI told me that he will contact my references and will get back to me within one week. Later, he sent me an email saying he offered the job for someone else. Before the interview, he was writing me and even shared some confidential information even before they released them through the HR. 

The way he handled my application gave me very high hopes on getting the job. Now I suspect that my co-chair who was known to my future PI changed the post-doc hiring committees view of me. He has given me very good reference letters in the past for other academic positions that I can start in several months if I was selected. I am now confused, and not sure how to handle this supervisor. Any help/advice greatly appreciated!
This is a common reason why job references are normally asked from the former employers but not from the current employer. If the current employer would like to keep a specialist, adequate recommendations just cannot be provided because of the conflict of interest.

Try to get alternative recommendations. Maybe your work included cooperation with other laboratories, or maybe you had some more junior "intermediate" supervisor, or maybe you have worked along speciality at least part time? Suggest these people instead. 

In the worst case, you may even ask for recommendation from another co-worker like you. Not very good yet better than nothing.