She cannot adopt him without your parental rights being terminated - this is WAY different than custody. Your ex-husband - not his wife - would need to have his lawyer file papers claiming you to be unfit and asking the courts to terminate your rights. Judges are VERY reluctant to terminate parental rights on parents who are addicted to drugs, much less one who isn't.
Look in your area for a legal aid society, they may have a list of lawyers who would represent you for a lower cost, or maybe even free. You need to have the Court order visitation - not just take whatever your ex-husband gives you. If he won custody in court, there should have been some kind of order as to what your visitation is. He cannot keep you from that if it is court ordered.
This may require you being very bold and stepping out of your comfort zone. You will need to decide if you are going to fight for your son or not. I would personally encourage you to do absolutely everything you can to maintain a relationship with your son and under no circumstances should you voluntarily sign over your rights.
I am a legal assistant in the area of family law. I don't know everything about your situation or your state laws, but I know that she cannot adopt him without your rights being terminated first. That can happen two ways, one - you sign over your rights (REALLY bad idea) or two - your ex-husband proves there is something in your life that is extremely detrimental to the child (this can be VERY difficult to prove). So, my advice is to stand up and take action in order to keep your son.