We had an orthodox chupa ceremony, because my ex lied about being Jewish to the local Chabad center. Now I investigated and found out the the only Jewish person in his family was his great grandfather. So in this case, do I still need a Get from him or our marriage in not valid?

I am very sorry to hear of this deception.

If your ex is not Jewish, no get is required. However, it is very much in your best interest to obtain an official letter from a beit din certifying that no get was required.  When you seek to marry a Jew in the future, the rabbi will require either the get or that kind of proof.  If your ex is willing to cooperate, it might be simplest to receive a get anyway.

I'm happy to facilitate a connection with a beit din if you'd like.  I'd have to know your location to set that up, or alternatively you could contact the Beit Din of America directly and tell them I suggested you call.

I welcome follow-up questions and please tell me if you contact a beit din and what your experience is of their process.