In 2013, Bush told Candy Crowley that "I think people will respect the resolve that my brother showed, both in defending the country and the war in Iraq." Why would we expect him now to lose respect for his brother? That wouldn't be a show of resolve worthy of the Bush name.
In 2014, Bush argued that the United States should keep 5,000 to 10,000 troops in Iraq as it has kept troops in South Korea since the Korean War. But more, he specifically said that "we should never say that we're leaving because it's important for a political campaign or it's important for domestic purposes. To leave, to do it in the right way based on our security interests is fine. But to do so simply because it is politically expedient is the wrong thing to do." So too would saying, because it is politically expedient, that he would make a different choice on Iraq knowing what we know now be the wrong thing to do.
During this campaign, in recent month, Bush has similarly called for the U.S. to "reengage with some small force level who can help continue to train the Iraqi army, to be able to provide some stability."
That's quite a record of Jeb Bush going on the record in favor of military intervention in Iraq. He thinks his brother showed resolve in Iraq. He thinks the U.S. should have ignored the differences between South Korea and Iraq and treated Iraq like South Korea, and that opposing military action because it's politically expedient is the wrong thing to do. And he thinks President Obama should be sending more troops now. So, when he's asked "Knowing what we know now, would you have authorized the invasion," why would his answer change?