As the USA minister of defence, he is in all likelihood responsible for crimes against humanity/war crimes, for instance, the killing of people on small boats in the Caribbean, and the hundreds of school girls in Iran, to name just two cases. But who could sue him; it seems it is usually countries that do this, but could eg an organization of international lawyers do this? What would it take for an individual to get him sued? (Asking for a friend).
They recognized it in 2002 when, in response to its formation, they enacted a law enabling the president to free americans and certain allied individuals from being detained by it, and prohibiting cooperation: https://en.wikipedia.org/wiki/American_Service-Members'_Protection_Act