Honestly OP, it's a terrible idea for all the reasons you outlined in your previous comments.
For transparency, I have two DC who are autistic and have various associated disorders. I suspect DD of having ADHD but she's not diagnosed with that, just autism and language processing disorder.
I am also autistic and have ADHD and let me tell you now, transitions are a bastard. I can sit for an hour in the bathroom trying to get in the shower and not being able to do it because well, who the fuck knows.....it just doesn't feel "right" even though I really want to.
I have children, a good career, a long term relationship, a mortgage, a dog.....you get the idea. And yet sometimes I can't manage to get a drink or feed myself for literally hours because transitions....
As a PP mentioned (sorry, forgot your name!) time blindness is a real difficulty with ADHD. LOTS of regular (calm) reminders and countdowns will help to break the difficult with getting moving. It needs to be more often than every 5 minutes for a 6 yr old. Very frequent reminders will be disruptors and can provide sufficient motivation to be able to move.
My DS and DD present very differently. DD struggles more with transitions and needs extra time to get her butt to the table. Can you factor this in? Start calling him earlier? Allow him to bring the tablet to the table until his plate is in front of him?
Also, my top, top tip is don't get drawn into discussions, disagreements etc. You can let him know that dinner is 4 minutes away, and then walk away. If he's cross, you don't need to engage with it. Deliver the information in a calm and detached way, with no judgement regardless of his response. Walk away. Back one minute later "dinner is three minutes away - can you come to the table please?" And so on. Calm, non-reactive, non-emotional, non-judgemental delivery of information. He's over-stimulated and overwrought and you need to be the predictable, calm rock face that's consistent but clear.
You might want to consider visual aids too. Lots of visual aid systems include either a traffic light system or a smiley/sad/neutral face system that the child can use to communicate how they are feeling. A visual timetable might also help with dinnertime - you could use it to show the countdown and when he needs to move. Visual aids aren't just about communicating for understanding - they're extremely reassuring to children with autism/ADHD and provide a tangible anchor. For ADHD kids the time blindness can be such a problem that a visual timetable can really help to ease stress.
Sorry, bit of a brain dump here! I could go on but don't want to create the world's longest post haha! Sounds like you're doing a really good job OP - the talking about the game as a means of distraction and refocusing his attention is an excellent approach. Much better than the code word!!!