Child support

Hi! I would like to seek for an advise. My ex- Bf just stopped sending his financial support to our son. We both agreed that he will send me money twice a month once he gets paid and he will shoulder the half of my hospital bills when I give birth plus he will provide a baby crib, too. When I gave birth just last March 2012 he only sometimes sends me money and never shoulder the hospital bills and baby crib. And just recently I realized that he changed his numbers and now I dont have contact to my ex. We have a written agreement already about his financial support and we have a witness too when we both signed it. My question, do I have a chance to sue him for not providing financial support? If yes, what are the steps so I can file this asap. Can I use the documents that he signed against him since he failed to do so? Do I need a lawyer for this to file a case against my ex Bf? I cant afford for a lawyer, do we have like government lawyers who can help me for this if I have a chance to sue him? Thanks