While I had explanations for the apparent contradictions in my previous CS, I'll make something different.
(Due to the amount of blood in him, most of his mass was taken up by veins and blood, thus resulting in more fragile everything. Everyone with his bloodline ability actually does suck at ninjutsu; he's just slightly above the norm with it. Still, chances are, he'd just start vomiting blood if he tried to use it.)