(immigration ?)My son is in mexico, how do I get him back?

I went to mexico oct. Of 2008. I went on a mexican bus. I had no passport, and at the border didnt even get asked my name. No questions, I was in tijuana. While in mexico I found out I was pregnant. While in mexico, my relationship turned bad (with my bf who I went there with) its a long story, but he didnt let me come back to the us to have my son. A week after my son was born, he left me and my two children. Where he went I do not kno. I stayed with his mother. I tried to register my son but since I had to do so as a single mother, and had no passport they wouldnt let me. So I got a job hoping to sace enough money to get my passporr, but it was almost impossible. I made 900 pesos a week and that barely covered expenses. So a boss of mine bought my ticket back home. But I had to leave my youngest son. And I tried getting my oldest son a passport, but I need his fathers signature. Its a big mess. I need to get my son home. My question is: is there anyway I could do the process from here? And then go get him? Because I want to spend the least time there that I can. any help will be appreciated, thanks.