Can i adopt my girlfriends son that was taken by cps for neglect (drugs in her system) so he is not in foster care?

You do have legal rights and you need to speak with an attorney ASAP.  You can ask the courts to find that you are the child's father by filing a petition to establish paternity and informing the court of your contention that you qualify as a presumed father under Family Code section 7611 (a presumed father is one who "receives the child into his or her home and openly holds out the child as his or her natural child").  If you have a good relationship with the mother you can also file a voluntary declaration of paternity with the Department of Child Support Services which will serve as a judgment that you are the child's father.   Let me please point out one other thing.  The baby was not kidnapped.  The baby was an infant being raised by a mother on meth.  CPS did the right thing here, and it is not going to do you, the baby, or your girlfriend any favors to minimize her role in this situation.  Children of meth users can have serious complications.  This isn't do discourage you from coming forward as the father, you should do so if you love this child and can provide for his care.  You just need to be realistic about the situation. Good luck to you both.