I think I found it.
*Virtual War* by Gloria Skurzynski

From the reviews:
> He has been genetically engineered to have fast reflexes so he and his team can win the virtual war...
>
> His days and nights are spent in a box, with virtual walls...
>
> Three genetically-engineered young people have been raised and trained
> their entire life to fight the war: Corgan with super precision,
> dexterity, and timing...

https://www.goodreads.com/book/show/2211941.Virtual_War