The prevailing legal opinion is that an executive order would not be effective in defining the words "and subject to the jurisdiction thereof" found in the 14th Amendment (largely because Congress has already legislated in relation to this matter). However, there are a number of legal experts who believe that Congress has the power to define that phrase in such a way that "birthright" citizenship would no longer be recognized (without a constitutional amendment). The fact that no previous congress has defined it in such a way, and the additional fact that a number of statutes which recognize the existing practice would also have to be amended, make the legal proposition that SCOTUS would accept redefinition by congress a 50/50 proposition at best. However, the political prospects of this happening are far less likely. Even if the Republicans controlled both the House and the Senate, a number of Republican representatives would be unlikely to support the legislation required.
I don't find Trumps idea here to be "stupid". It's just highly unlikely that he can get enough political support to take a realistic run at it.