Trump can't unilaterally order the release of the memo because it's comes out of another branch of government. Congress will ask him to sign off on it for the executive branch though.
As I write this, it is 6:45PM EST. Nunes was going to call for a vote at 5:00 PM EST, so the outcome may already be known. And, of course, Trump will probably sign off immediately.
I'm betting they did it this way to avoid that Shifty-Eyed Schiff and the democRAT blowback from getting any press before the SOTU address.
The memo could be public by tomorrow and everyone will know what's in it by the time the president gives his address to the nation. Boy, this is some political theater and intrigue!
And even Trump doesn't want to deal with that...