This year I tried to file my son on my taxes. Little to my knowledge did I know that someone had already claimed him on their taxes. The tax place that I went to did little help in my search for who could have gotten ahold of our information.
Do anyone know how I can find out who abused his information?
How can I get these people prosecuted?

When you attempt to file electronically, it will kick out the return if someone claimed on your return has been claimed on another return. You can still file by submitting a written form with a letter to the effect that anyone else claiming your son is not allowed to. Then let the IRS sort it out. It could be a simple error in entering the SSN.

If you think that you are the one who can claim the dependent, then file your tax return and claim the dependent. It you are unable to efile, then send paper return. IRS with then contact both of your and figure out who must amend the tax return and pay the tax due with interest and penalty.

Only custodial parent can claim the child. It does not count who is paying child support. Also, non-custodial parent can claim the child only if the custodial parent signs a release Form 8863 or there is a court order meeting the IRS requirements. Read: Your U.S. Tax Return: Child of separated or divorced parents