One heat pad under his warm hide would be enuff but you need to get a Thermostat to regulate the heat from the pad they can get to over 110f easily and that will burn your snake. I use a Vivarium 300 fron Reptile basics
http://www.reptilebasics.com/ve-300 And a Ranco both are working fine for me
You might also want to cover 3 sides of you tank with foil tape on the outside of the tank to help reflect heat in. NEVER put tape in your tank I have heard horror stories about this. Let me know if this help

One more thing Heatpads usually dont raise the ambient temp of the tank much if at all you may also need to get a heat bulb or Ceramic heat emmiter