Answer:
Exothermic.
Explanation:
Exothermic is when he leaves the system. A way to remember is the "ex" which is a prefix/root/suffix or something that pretty much means leaving, I think. For example how it's in "Exodus" and "Exit", I think.
Endothermic is when it goes into the system. Similar to "ex" but with inside, I think. For example how it's in "Endoskeleton", I suppose.
