Bearing in mind how much of the volume of the dish is rice and how the seafood, etc is spread throughout, I think accurately managing the amount of rice you eat could be a bit of a challenge.
You could try your son on the pretendy? My OH likes it, but prefers the real thing, which is why I mainly do the pretendy if he's out or away, but he doesn't walk away from the plate if it is presented to him with cauli rice.
Or alternatively again, use a small amount of rice, so that the proportions look a bit wonky to begin with, then at the end, add the cauli rice and treat as before. Of course, that might disturb any of the cooked on deliciousness at the bottom of the pan, but it's an option to reduce the carb count substantially, if not to the full potential extent.
Just a thought.
(You can probably tell I enjoy paella - great flavours, but to be fair, in my world, almost anything involving chorizo is a winner.
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