get a lawyer and find out your rights and press for visitation/parental time, whatever it is they call it these days.
make sure that whatever action you take, you take it out of love for your son, NOT out of spite for his mother. do NOT get angry, do NOT get vengeful, do NOT let your emotions get the best of you. fight for your rights as a parent because you love your son, not because you want to get back at her. make YOUR actions right. make YOUR intentions loving and respectful. then its not going to matter what her reasons are, eventually, they will bite her in the butt all on their own without you having to actually do anything.
courts are hesitant to take away biological parent's rights. so im betting just a legal arrangement will solve any issues. she cant legally keep him from you. and i dont think that the adoption can go through without your signature anyway. (unless shes going to claim you died or she didnt know you or something).
anyway, make sure first and foremost that your intentions are pure of heart and in love. anything less than that ..... you will have a hard time.
good luck.