The President has NOT invoked executive privilege, and Sessions doesn't have the power to invoke it. Sessions and Trump are engaged in a desperate attempt to run out the clock on Mueller's investigation, and that strategy will not end well for either of them.

Senator Whitehouse stated, “Contrary to Department of Justice Department policy, Attorney General Sessions doubled down on his non-assertion assertion of executive privilege. It has been four months since he refused to answer questions before the Senate Select Committee on Intelligence.  How much time does he need to get a response from the White House as to whether the President asserts executive privilege? The Attorney General had months to approach the President about whether he’d make such a claim, and we gave him plenty of warning that he’d need to address this today. This is a stonewall, plain and simple"