So when you give him a warning that you see he's ramping up...
what happens next? Does he have something he can do that will work to get calmed down? Does he ever notice he's getting ramped up?

One of the most helpful things here was getting the kid to start to notice the signs he was ramping up - increased heart rate, muscles feeling tense, negative self talk. Once he got that he was much more open to learning strategies to redirect himself.

I'm wondering if your son is engaging in a lot of negative thinking (that he's not expressing) so when you come in and offer a suggestion he's hearing something much more negative because that's what he's been thinking about himself already. Does that make sense?