the president can't fix the economy any better than the soviet union could centrally manage theirs successfully, it is not possible to reach economic prosperity by having someone with a gun telling everyone how much to produce of everything.
the way modern economies work today is broken.
the main problem is that we no longer have free markets, mainly because central banks try to centrally control the amount of money and credit in circulation by setting an arbitrary interest rate, instead of letting the market decide what the interest should be.
the president could in theory abolish the central bank, and stop the commercial banks from conducting fractional reserve lending, and stop propping up failed businesses, which is why he is somewhat responsible for this mess.
you cant blame only the president for this mess, you have to blame the whole government from this mess, since the time they started taking loans from banks and printing more money, and these caused the USA to lose there rating from owing not only banks but other countries as well.