Advice on 8yo using threatening language

My 8yo has ADHD and Autism, the ADHD is more profound but there is a lot of overlap.

Recently there have been a couple of incidents where he has used language like ‘if you don’t….then I’ll’ And then clench his fists or mime punching someone.  

Today during a martial arts class he told a child to get up to perform an exercise they had been taught or he would ‘stab him’. The child understandably was upset and I told him off for being unkind, demanding he apologise and stop being nasty.  I later explained using that language is very cruel and dangerous and to show me what he thought ‘stabbing’ meant. He jabbed his big toe in my direction. He couldn’t tell me where he’d heard it from.

I then gave him more context and explained using this language can get us into serious trouble, but is there any advice for me as a single parent trying to help him understand better? He raves about sone poppy playtime game (we don’t have it) without making excuses I wonder whether this has anything to do with it? 

Generally he’s a kind little boy but I really want to address this behaviour sooner rather than later and don’t want to brush this sort of thing under the carpet.

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