- He can let the parallel case in Boston work its way through and the matter will need to be adjudicated at the Supreme Court, where he will have an excellent chance to prevail; or
- He can alter the order slightly and ram it home again; or
- He can change course.
He won't take the latter approach, but either of the first two will work. As John Hinderaker points out, the relevant statute makes it clear that Trump has the power to take action:
Remarkably, the Ninth Circuit decision fails ever to mention the relevant portion of the Immigration and Nationality Act, 8 U.S.C. §1182(f), which provides:
(f) Whenever the President finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may by proclamation, and for such period as he shall deem necessary, suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate.
That's about as categorical as it gets. Unless the plaintiffs in this case are prepared to argue that the Immgration and Nationality Act is unconstitutional, which they have not, Trump will eventually prevail. I'm betting Trump will pick the second option.