I guess it would be reverse Atkins ...

Considering he's a bit underweight .. I'd imagine I'd start him off as if he's on the final stage of Atkins ... high up on the carb ladder ... plenty of good carbs (use Ketostix to ensure he's in ketogenic state) Get him so he starts gaining weight then start scaling back the carbs until his weight gain slows .. then eventually stops - bam he has his target carb count.

Why? Two reasons.
First is that his eating habits as they are now, are horrible. He is picky and doesn't eat foods he hates - then snacks out on all the bad foods whenever he can.
Second, and mainly, is because of attention and focus problems. School system and early development assessment groups have already been micro-monitoring him - he is in queue for neurodevelopmental tests... I'm thinking it can't hurt .. especially since Ketogenic diets have shown to stabilize neurological issues (MS, Epilepsy)

I know myself, since getting on Atkins, have noticed my ability to focus at work DRASTICALLY improve. Clear chain of thought and alert.