Coaxial cable connection for high definition

My dad has a coaxial cable in the wall between the Cox box and the TV connection. When the Cox technician installed Cox service for him recently, he ignored the coaxial cable and hung a HDMI cable down the wall between the box and the TV. I tried hooking the box and TV to that coaxial connection and cannot find a signal in any of my TV input options. (1) Can I use a coaxial cable between the box and TV? (2) If I do so, can he still get high def?