That is an EXTREME burn. It does seem strange that the heating pad is no longer that hot, but also I think the temp can flucuate, and it does make it seem like something intentional, but also it's on his belly which indicates that yes it was the unregulated UTH.
Out of curiosity I measured the temp of an unregulated UTH with a temp gun and temps were reaching 120 degrees F without direct contact (I tested it underneath a tub so get real life temps)