I think it worries me that he DID say it in the middle of a melt down, because that is when he is least in control of his own actions, and most likely to impulsively act on a new idea like that.
I guess the plan I would propose is to get him recognizing the path to meltdowns and halting them. If he can be that extreme within a meltdown, I wouldn't want him having them. I know it's hard, because the triggers can be so invisible, but he needs to learn to self-calm so he doesn't get in to those situations.
We've had a lot of threads on that before, and some great insights from AS kids on what their meltdowns feel like to them. If you can search out those, it will help quite a bit.
Otherwise, in my house, it is my NT daughter who throws that phrase around and, while I recognize that in part she simply can't verbalize her extreme feelings, I do take it VERY seriously that she can even imagine saying such a thing. We've talked about it a LOT, but the reality is she can be very depressed, and how to solve such deep depression in an 8 year old is tricky. I'm not running around in a panic thinking she is about to act on it, but I AM trying to figure out how to deal with her moods and get her on a more positive track. She isn't just trying to push my buttons with it, that I am sure.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).