If the employees working directly for him are called his direct reports, how should one call the complete hierarchy of employees who work for him?
The MLM term "downline" means exactly this, but it's frequently used in conjunction with pyramid schemes and the like. "Subordinates" is possible, but that could suggest people below him in other branches and thus not under him. You can use the term "span of control" too by describing them as "employees in his span of control".
If you're going to say "direct reports", why not "direct and indirect reports" or just "reports"?