Most of you know that I am no Trump cheerleader, nor am I an NT (since Nov 2016, anyway), but this is a good move, and he wins politically, no matter the outcome.
(1) Issues the EO, where the Supreme court has to look at the meaning of "subject to the jurisdiction thereof" for the first time since 1898, and may agree that this does not aplly to people who enter here illegally.
(2) Issues the EO, SCOTUS may say Trump cannot do this, not rule other than that, but may say congress can by statute, which still gives Trump a political victory, and moves the ball into Congress's court.
(3) Decides not to issue the EO, but uses the bully pulpit to rally people to contact members of congress to take action, again another political victory.
(4) SCOTUS may rule that this needs an amendment to over turn, which would be difficult, if not impossible to get, but cost Trump no political Capital for trying.