I thought the weapon looked odd, especially if it was purported to be a double barrel shotgun. Looks like an exposed hammer... And a large front sight. Seemed odd.
But now I wonder if those might not be artifacts within the photo of background behind the gun. The thing that looks like a hammer might just be something on the keffiyeh scarf, not the gun, and the same with the spot that looks like a sight on the end of the barrel. It might just be a dark spot on the wall behind the gun. At that resolution it’s difficult to say. Without those features the gun could easily be a double barrel shotgun.
Looks like a lever gun to me. He is holding his 2 smaller fingers outside the lever to give him more purchase on it for quick actuation.
Caliber? Who knows. 30-30 is most common but there are a bunch more.