He wants something different than the post 9-11 War on Terror authorizing resolution.
Since that resolution gives him all sorts of power to prosecute the War on Terror (of which ISIS is a part, and a defined enemy) there shouldn’t be a need for another one. So speculation is that since the Bush Admin put in place and set precedents for certain constraints, he wants something with his own signature on that he can then have his advisors intrepret. Not having to defend reinterpreting the 9-11 authorization.
the real reason is that he wants other parties to spread the blame around if his “plan” doesn’t work.
He wants something he can reasonably interpret as authorization to overthrow Assad.
And engage Russia militarily if that is necessary to accomplish the Assad overthrow.