Doesn't look too bad personally. The way you got the probe set now will work but you can also tape the probe directly to the back of the heat pad also making sure not to tape the end of the probe. Then you want to use either a thermometer or better yet a temp gun and dial in the temp while taking the reading directly off the bottom of the cage, not the substrate as the snake will burrow and push the substrate aside to get to the heat. I probably would put the guy on paper towels as substrate for a week so you can monitor his belly. As long as it doesn't look like its getting infected or splitting or anything funky like that, then you can put him back on his usual substrate.