where the brain plays an active role.
Screen time drains unfocused mental energy but the brain plays a passive role.
His focus can only sustain a limited period of time, then it needs rest by shifting into that passive role. The brain alternates between the two roles, thus taking longer time to finish the homework.
Let him know this might help.
There are brain-teaser books. Have him work on the problems to train to extend focus time. Do them outside of school work.