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).
He could be, and what it will look like is absolutely nothing. The MAGA cabal is untouchable. They are above the law. And since no one is willing to do what is necessary in these situations, they will remain so.