Assuming he has been with you long enough to file, you can file a paternity case or child custody case and ask for managing conservatorship. Your girlfriend will need to be served or sign a waiver of service, and the court can then decide who your son stays primarily with, and address other issues like visitation, child support, and health insurance. Until you file a court case, you will remain in limbo because she has as much right to step back in and take possession and you have to keep possession (and she has even more rights if you have never had your paternity established). I highly recommend you file with the court and ask for custody soon.
