26 USC 6103(g) Allows the president to direct the IRS in writing to provide return information to certain federal employees he designates. 6103(g)(3) then allows that employee to disclose that information to any person (emphasis person, not employee) the president directs them to in writing.
President Obama should direct the IRS in writing to give Donald Trump's return information to a Federal employee, who he then directs in writing to share with enough journalists to make the material public.