He started the process of rescheduling. The HHS recommended schedule 3, and we’re awaiting DEA approval. You can thank Congress for tying the DEA’s funding to their arrest count for the stalling. Independently of rescheduling, Congress alone can roll back federal regulations to decriminalize possession, but half of them prefer having full prisons.
The only other way he could override the law is by Executive Order. Those are temporary orders meant to address a crisis while proper legislation is put in place. Issuing an Executive Order without certainty of follow-up legislation would be extremely disruptive to the cannabis industry when it expires.