I feed all of my snakes in their enclosures and have no issues with cage aggression. I do, however, hook train... That is, I always touch the snakes with a hook before I handle them. They now associate that with being handled and come out of "food mode" when I touch them with the hook.

Maybe he just has a boa with a bad temperament? I would just handle any boa you're considering before purchase to make sure it isn't aggressive. None of mine have been nippy, even as little worms.