The military takes samples for DNA testing during in processing, so they already have his DNA or the ability to test a blood sample for it. But they said they were going to wait to get a sample from him when he's brought back, so that there is no question about the source of the sample. (You can imagine what a defense lawyer could say about a blood sample taken at the same place on the same day as hundreds of others and stored for a couple of years.)
What I do not understand is why it is the military not Onslow County or the State of North Carolina that seems to be in charge of getting the DNA results. Maybe they can get results faster than the state? With all the suspicion surrounding the Marine Corps' handling of this case (which I don't necessarily share, personally) it just doesn't seem wise to have this forensic evidence coming from them.