Regardless of his dietary or workout routine, his arms are fairly large and disproportionate in relation to his waist. It can happen, but it is unlikely. Usually, big-boned guys can get arms that size, but big-boned guys don't usually have such small waists. Just as it is possible for a slim, small-boned woman to have large breasts, you wouldn't likely bet that they were natural.
I assure you as with much of my family the giant "swimmers build" can happen naturally. He might have had some lipo in the abdomen or something. Also swimming of course. It's however easy to get the specific smaller muscles with EMS if you put foam in a drink bottle top, normal water to wet it and a small hole for the electrode. Just slide it around until you feel the exact point to get the deeper abdominal muscles contracting. This flattens the gut hard without adding muscle bulk. He was your typical looking heavier big lad before anyway. At 3:16.
Well Frederick, two things I'm good at without any real effort, shooting and being fit looking. Two things that result in never quiet agreeing on anything? Shooting e.g. few know that the AR in AR-15 stands for Actually Rifle as in "Well actually.." and fitness is similar hehe