Yes. All classified information must be classified by what is know as an Original Classification Authority. However, the authority of these individuals to classify that information is derived directly from the President, as was most recently laid out in Executive Order 13526. Along with their ability to classify information comes the ability for them to declassify that information, or, as laid out in section 3.1.b.(3), someone in a supervisory position to them who is also an Original Classification Authority can also declassify that information. Therefore, the President, being the source of classification authority and acting as a supervisor for any delegation of Original Classification Authority, could declassify any piece of information he wished.