Possible body modification for unusually large sword wielding
<PERSON> was a cook, a really strange one. He would walk crooked forward and slightly bent to his right. From time to time I saw him loosing his balance trying to speed up the dinner time, only to be rescued from falling by nearby sturdy objects which he could grasp on. I never really saw him falling, but that would be a show of its own due to his apparent heavy weight. One time I saw him carrying alone one large pig on his right arm and wondered why he seemed so straight and on balance. Anyone trying to carry that pig would likely drag him through the ground. Even if I was strong enough to lift that animal from the ground, I would definitely fall face first on the mud, that thing was huge!
He has something like a stump on his left side back, near his shoulder. Everyone says it's a callus, but I never saw one so big. 722
He would never take off his shirt in front of anyone, so nobody could be sure. <PERSON> said it was the reason he could fight with that monstrosity of a sword. When I first came to the camp I thought that was a cleaver to butcher bulls. I'm responsible for oiling it before battle, alongside <PERSON>. It weighs more than I do and it takes both of us to tend to the blade without ruining it by dropping it on the stone floor of the barracks.
I never watched him battle on a real fight, but one time he sparred the captain. It was incredible how on balance and fast he was wielding that blade. I don't think something that big should move that fast, the gods shouldn't allow that.
TL;DR
<PERSON> has a growth on his left side back, near his shoulder blade. In the story there are people with steel skins and it will be revealed later. The growth is made from solid steel, explaining its massive weight.
-He is stronger than an average human, nothing like superman power, but something like 4 man-power units.
-He compensates it by leaning forward, but is really clumsy and loses his balance frequently.
-While holding something heavy on his right hand, his center of mass returns to that of a normal human being and he can move and fight really well.
What I need to know
-How believable is that? I know that with fantasy anything could work, but being believable is preferable.
-What other body modifications would he need to have in order to wield a really heavy and long sword? I'm thinking about having his lower body heavier and also with 300