The lawfulness of a military order is a matter of law for the MJ to decide. Lakin made his case. It was predictably uncompelling and was rejected during pre-trial motions, and the rejection was well founded in military law.
He's not going to "file suit" over anything. He will have an automatic appeal to ACCA. That chances of him prevailing on appeal are the same as his chances of prevailing at GCM - precisely 0.00%. The chances of CAAF granting review on this case is slightly better - 0.001%. Once they refuse to grant review, it's over. That's it. Of course, Lakin isn't going to be confined for nearly the requisite time for a habeas petition.
It can also be an essential element that can be presented for a jury to decide. Either way, an accused is still supposed to be able to challenge lawfulness in the trial.