My return was rejected by IRS saying someone used my son's SSN and filed a return with it.

If you have a child old enough to have an after school job, etc., then they are usually the “first suspect” for this problem.  Make sure your child did not “claim himself” on his tax return.  Or…… 

Someone else claimed my dependent and my return was rejected

If someone else claimed your dependent already, then your e-filed return will be rejected.  Print, sign and mail your own return which claims your dependent.  Mailed returns are processed differently by the IRS.  It will take some time (maybe even up to a year) but eventually the IRS will contact both parties and sort out who could rightfully claim the dependent.  The person who wrongfully claimed the dependent will face repayment of refund $, plus penalties.