Does Obama need it? Or does the 10 year old Authorization for the Used of Military Force (AUMF) that George Bush obtained to overthrow Saddam Hussein cover zapping ISIS in 2014. Who knows? Call in the lawyers and let them quibble.
Let's get real about the issue here. Obama is commander in chief, the armed forces will do what he orders them to. So Obama can order up air strikes, or anything else, and it will happen.
On the other hand, any president with two brain cells firing, wants to go to war, or something less than war, with Congressional, political and media support. Obama has already gotten funding for the anti ISIS effort into the "continuing resolution" a stop gap bill to fund the entire US government at least thru the new year. That's not too shabby for Obama. That will keep the bombs falling for a while.
If Obama isn't very serious about ISIS, and will settle for a few fireworks shows, no ground troops, that's probably all he needs. If he actually wants to zap ISIS into the next dimension, he probably wants to have a brand new AUMF that specifically names ISIS as the enemy.
To my way of thinking, there are only two reasons for NOT going to Congress for a new AUMF. One, Obama fears that Congress would vote it down, making him look weaker than he already looks. Two, Obama isn't really serious about ISIS, and doesn't actually plan on serious operations to wipe them out. Take your pick.