How can I get sole custody of my son if the father hasn't been involved?

If there is already an order in place you would need to seek to modify the existing order for conservatorship, visitation and support.  If there is not already an order you would need to file a Petition in Suit Affecting Parent-Child Relationship to establish conservatorship, visitation and support orders for the first time.  However, if a step-parent adoption is your goal then it may be more efficient to talk to an attorney about the possibility of moving forward with step-parent adoption if there is grounds to do so.  I highly recommend that you consult with an experienced family law attorney in your area to determine what the best course is for you and your family moving forward.