can you fight retroactive ,when you are being charged for two years,but i didnt know he was my son until mother did dna test after those two years ?

  The statute lets the court go back 24 months from the date the petition was filed.  It doesn't matter whether you knew about the child or not.   Sorry, you are on the hook.  The good news is usually the courts let you pay the retroactive support over time, and add 20% of the current monthly amount to the arrears. Cindy Vova