Heating rocks and heating pads are dangerous for bearded dragons. It is urgent that you remove both of those items from his tank immediately. Heat pads and rocks have been known to cause severe burns to the undersides/bellies of bearded dragons and other basking lizards. They are basking lizards who require heat from above, through lights, and not from below. They do not sense heat through their bellies and will not be able to tell if the heat rock/pad has become too hot, resulting in debilitating and painful burns.
Since they are basking lizards, they need heat from above in the form of a light. These lights do not just give off heat, though. They are also responsible for the synthesis of vitamin D3 in the skin. The vitamin D goes through a few chemical changes in the skin and the organs, and eventually goes into the bloodstream, supplying the rest of the body and allowing the body to properly use calcium in the organs, such as the liver and the heart. Proper heat is required for this chemical change to occur.
There is a complex relationship between the UVB lighting, calcium and vitamin D3 that all rely on your dragon receiving proper heat in the form of basking lights.
If you would like to know if your own bearded dragon set up is optimal, please post a picture of his viv, and a description of the kind/brand and types of UVB lights and basking lights that you use, thermometer kind that you currently have, and the temperatures directly under his basking light, and the temp of the cool side of the tank. We can advise you and help you out.