Sessions is putting it back where it belongs - the Congress can either change the laws or accept that laws have consequences - they cannot blithely allow them to stand and be ignored for political convenience.
What the executive branch can do without assistance from the legislative branch is change marijuana from a Schedule I drug to a Schedule III or IV drug. Schedule I drugs are defined as drugs which have no currently accepted medical use in the United States, a lack of accepted safety for use under medical supervision, and a high potential for abuse.
There seems to be some reasonably strong evidence that marijuana does have currently acceptable medical use.
Once a drug gets off of the Schedule I list, it can at least be prescribed under some circumstances.