But so many meltdowns and issues could be avoided if he just vocalized what was wrong.
Don't know if this fits for your DS, but for mine, the issue seems to be that he doesn't actually recognize what is wrong when under emotional siege, but can later put it all together and communicate. He does need help with this, though (in the form of questions, suggestions for describing what's happening...e.g. "is it like this? Or is it something different?")
It's a pretty common scenario for anxiety. One idea is to ask DS to deconstruct post-mortem. Can he describe any physiological reactions? If he can begin to identify when he begins to go off the rails, that's a good start. He needs some strategies for self-soothing. Ask him if he can observe himself as a "disinterested third party."
The meltdowns are kind of self-perpetuating because they cause more anxiety (child feels ashamed, child realizes he's stressed out everyone around him, etc.)
This would be a really good thing to work on in tandem with a therapist.