I hope I don't sound stupid right now...
thermal gradient, the difference between the highest temp in his house and the lowest, right? how would I know this if he is outdoors?
Yes. You've got the right idea. They need a warm end and cool end so they can move back and forth to regulate their body temperature and keep it where they want it.
Outdoors, the thermal gradient occurs with shade and sunny patches. They can also dig into root balls, or some species dig burrows, or russians just dig into the dirt, to thermoregulate.