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 already did that. Just recall the dialogue he has on phone. And giving that he's always telling the truth (in a very specific way) when talking with Petersons, he mentions going to Florida. On the other hand, his photo in military files doesn't match with his face, but the guy from the file is also nowhere 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
- The Photo in the military files looks exactly like David(https://youtu.be/5XupvPqevtY?t=49)
and it looks like the 'David' he points out in the squad photo on the wall of the Petersons living room. The angle of the picture of Dan Stevens, plus the lighting, and the fact we're too assume he was a patient at this point and may have been recovering from injuries (or already showing signs of mental break) mean he looks gaunt and much more ruffled and undone than David appears, but it is definitely the same face and the same person. Remember, Dan Stevens was incredibly skinny when shooting started as he'd recently played a drug addict, so that photo, which was likely taken at the start of shooting, shows him probably twenty pounds LIGHTER than he is for the rest of the movie. But its the same exact face, just skinnier.It could be argued that there was just some muddled writing, but that David has killed ONE plastic surgeon who WOULDNT do the procedures he asked for, and was calling another hoping he would do the surgeries.
- 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.