Then he obviously hasn't read the Authorization, which basically holds Iraq accountable for violating the cease fire agreement of 1991.
The GWBush authorization was against the government of Iraq that existed at that time.
Obama withdrew all troops, basically ending THAT war.
It would make no sense for him to return troops based on THAT war, since THAT government was defeated and replaced with a new elected government. If Obama uses the Bush Declaration of War, he would basically have to go after the existing government of Iraq, not ISIS/ISIL.
Calling all lawyers! A potential windfall may be coming soon.