I made a giant skeleton, and he looks fine, but he needs a helmet. Unfortunately, any time I give him one he refuses to wear it. It is set as an outfit item, the item has DefaultRace as the set race, and his armor race is also set to Default. Any ideas what might be going wrong?

Is he a dungeon denizen? If so, put the helmet next to him, link the helmet to the skellie via the "Linked Reference" tab, and put the script "DefaultAddToLinkOnLoadScript" on it, with both properties (ForceEquip and ShouldEquip) set to "True"