I don't know pigs from pig
poop, but is this trait all bad?
I mean, it seems like he might not get to mate at all, in which case you harvest him.
But if he does mate, you get a his size with his gentleness.
As long as he can forage, and fight for his life, his passiveness seems preferable.
Otherwise, maybe testosterone?
Seriously, I don't think you could train him to be tougher,but maybe someone who actually knows hogs will know better.