He sent the order to Burnside to move forward and force the Antietam.

It is said that at this moment Lee had only five thousand men with him, all the rest having been sent to Jackson, and, if so, time itself fought against the Union, as it was a full two hours before Burnside carried out his order and moved forward on the Antietam.
But Dick, on the north, did not know that it was as yet only cannon fire, and not the charge of troops to the south and west.

In truth, he knew little of his own part of the battle.

Once he was knocked down, but it was only the wind from a cannon ball, and when he sprang to his feet and drew a few long breaths he was as well as ever.
From muttered talk around him, talk that he could hear under the thunder of the battle, he learned that Sumner, who had come with the great reinforcement, was now leading the battle, with Hooker wounded and Mansfield dying.
Sumner, as brave and daring as any, had gathered twenty thousand men, and they were advancing in splendid order over the wreck of the dead and the dying, apparently an irresistible force.
Jackson, standing at the edge of a wood, saw the magnificent advance, and while the officers around him despaired, he did not think of awaiting the Northern attack, but prepared instead for an attack of his own.

There was word that McLaws and the Harper's Ferry men had come.
Jackson galloped to meet them, formed them quickly with his own, and then the Southern drums rolled out the charge.