My wife and I got married in 2013 and will be filing "married filing separately' when we do our federal tax returns. She claimed her son as a dependent on her federal return. His date of birth was 01/14/1988. He did not live with us in 2013. He was not married as of 12/31/2013. He was unemployed in 2013 and we paid more than half of his support in 2013. He is her son and my stepson. If we file "married filing separately" on our federal tax return do we both list him as our dependent on our return or should he be listed only once?

If you can claim him, you only list him ONCE.

However, his income MUST be less than $3,900 in order for you to be able to claim him, even if you did provide more than half of his total support.