Depends. If we are being realistic (going by the timeline) and it's 6 year-old Itachi, he would be smart enough to not just confront him. He would go to the village and inform them after gaining evidence of his existence like a previous user said. Then, Madara would be killed.
If you mean a 13 year old Itachi, I guess he could take on zetsu. Unless zetsu is controlling a body, he's not much of a combat type. That and Itachi would probably gain intel before attacking. Madara was just an old man who was hooked up to a tree, he was in no state to be fighting. Also, he wouldn't be able to manipulate Itachi, considering that Itachi has already gone through the most traumatic experiences in this manga and still remained sane w/o ever faltering. He never even gave in to the curse of hatred.
If it's 17 year old or 21 year old Itachi, he would definitely finish off zetsu and Madara himself.