"we had to take stronger measures against this disease in his mind that led him to put his younger siblings in danger of seeing something far more confusing and graphic than they're ready to at their ages."
This is what leaves me unconvinced (and I don't have expectation of changing your mind either - you operate with the most information here so that's probably a good thing). Certainly, what he did was inappropriate, but. . .a boy in puberty looking for porn on the internet is not a "disease". Even offensive porn. There's not something wrong with your boy. He did a bad thing (or a few bad things). He's a teen. Doesn't mean he should be let off the hook, but. . .diseased? Nah.
It does sound like you're having some conversations about this (and importantly, providing the idea that there's something better out there), though, so that's good.