Red doesn't appear at the Prison until you have investigated both radio towers (at Highpool and Ag Center). You will need to visit both locations and use the radio towers (attach one Repeater Unit to the first tower in the location you saved, and visit the tower in the other).
Once you've investigated both radio towers, General Vargas will contact you and tell you to visit the Prison to investigate how to access Damonta, which in turn will cause Red to show up and allow you to talk to him.