can i change my childs last name without the fathers signature since he is a registered sex offender and hasnt had visits in over 3 years

You need to file a motion, a request for order, and in your declaration explain the reason.  If you do not have access to a private attorney, go to your county courthouse and see the family court facilitator, who is an attorney who serves without fee to you, and can advise you as to how to go forward.