I'm sorry to sound a little negative here, but you need to step a little distance away from this situation. You have been with this woman and her child for three months. You are not his dad or his father figure at this point. You are her boyfriend! I understand that she's got issues with the dad, but you need to let her work them out.
The phrase "will I be able to use this in court against him? " is really inaccurate. You are not taking him to court. She is. The court does not take kindly to women have new "father figures" for their kids within three months of meeting them. Try if you can to give her and the dad some space to work out their custodial issues alone.
Again, I'm not trying to be mean, only realistic.
Good Luck,
Cali Stepmom