Son keeps threatening to leave the house how do I respond ?

[Grk]

His nearly 9 years but is very delayed with his speech social and emotional delayment acts a few years younger than he is . Diagnosed ADHD with potentially autism in the mix also
His not violent in the slightest towards us but is just very emotional anyway last few weeks whenever his not getting his own way that's it he says I'm leaving and finding a new family . After me just refusing him something this evening his got his shoes on and is heading towards the door .
Seriously how am I meant to respond to this in the best way.

I don’t have any suggestions but this is exactly what my brother did when he was 10. He’s undiagnosed but almost certainty ADHD and possibly autistic too - can’t control emotions and used to really really overreact about stuff growing up. One day after being told to share a toy with me and our sister, one he wasn’t even playing with. He put his shoes on and ran away. Turns out he Just cycled off on his bike to the next town, but this was 30 years ago, so no mobile phones etc. police were involved searching etc.

there was no acknowledgment or realisation at the time that he was ND, and honestly this has effected his entire life significantly. I think you need to not be too harsh on him, does he understand why he’s not getting his own way? Is this explained to him etc? Or is it more of a “because I said so” approach

i hope you get some good advice from other people xx