David is Caleb.
We know he underwent plastic surgery of some kind, and that never seems to enter into the plot. We know he isn't the real David Collins who served with Caleb, but he claims that he did "know" Caleb. If you accept that he did all of this to his own family the movie becomes real Nightmare Fuel.- The film strongly suggests "David" is going to change his face, not that he already did that. Just recall the dialogue he has on the phone. And given that he's always telling the truth (in a very specific way) when talking with the Petersons, he mentions going to Florida. On the other hand, his photo in the military files doesn't match with his face, but the guy from the file is also nowhere to be seen in the picture hanging in the Petersons house. Also, Major Carver has the right photo to present to Anna, so most likely somewhere someone fumbled something during the production.
- OR the photo in the military files is replaced with some randomly selected Ridiculously Average Guy due to being classified, while "David" was going to get a face job.