The king may, dunno, have some ability that can change the formation of a unit group to give it an attack/defense bonus and a speed penalty, like "Phalanx" or something like that. But probably Leadership is an aura so I don't think you want another "buffer" ability.
That said, it's difficult to find a suitable ability for it if you don't tell us what the abilities he already has do. Please elaborate on its role and skillset. I guess it's going to have a melee-supporter role ala Paladin + a strong "commander" flavour, right?