I filed petition & served my ex-husband December 2009. He has not filed a Response to my petition and any attempt for mediation has been met with defensiveness and more stalling on his part. I can't even set a court date because he never filed a Response. I was told the only thing I could do is file a Default Judgement on him.

In June 2012, I hired an attorney to just complete and file my Default Judgement paperwork for the case. I got a text from my ex today saying that he spoke to our mediator and was told that he or the mediator could "easily file paperwork to have the default set aside and start all over".

Is this possible? Can he just drag this on longer?

If he is trying to stall. You need time limits on your paperwork. If those limits are exceeded without contact then you file for a motion to compell. As part of discovery. Then when that due date has passed if nothing has happened you notify the courts so he can either be held in contempt or start signing papers. It wastes the courts energy if he delays without good reason. Then file for default.

A mediator can do nothing in your case they are only there for recommendations and to guide you through a process.

If it was for a child custody issue then the mediation is only a one time deal and that should have been over with long ago.

Most likely ex never talked to anyone. Mediators are that, they do not give legal advice. He would need to file a motion to request your default be set aside for specific reasons.