I was devorced in 1998. We had joint custody of all children. I had physical custody. My ex. And I split the depends. In August of 2009 my ex. Took me to court to longer pay support for our 21 year old son who was in college. I have supported my son 100% since, including all college related expenses, etc. The ex. Claimed son on his 2011 taxes therefore I am not able to electronically file my taxes claiming my son because he has already been claimed. What can I do?

First, your son had to be a FULL time student in 2010 for you to be able to claim him, and he had to be under 24 at the end of the year. (Neither is clear from the way you write it.)
If that's true, I would file on paper and attach a copy of the court order, and a big clearly printed note that YOU get to claim him, NOT your ex. Let the IRS charge him extra tax and penalty.

AT this point you have no choice but to file a paper return and explain this. ** that is assuming in the child custody agreement it does not give him permission to do it.

Thank you for the quick reply. Yes my son is a full time college student. He is under 24.