The undead that you summon is a temporary battle pet, so you can use the "Undead Command" skill with him.
This skill will works with ANY battle pet. It can be the boar from Horc Evader for example.
First off, you need to unequip any pet that you're using, then you use the "Summon Minion" skill, if you don't summon the Skeleton, use the skill again until you summo it(Sometimes a bug happen and you don't summon him).