Some private high schools will give assignments that in totality, really do take many hours per day since they require lots of repetition and formula application, rather than say, a small number of very difficult prompts.

In response, here are some "deals" that my friends cut with their teachers:

Friend 1: Instead of doing ~50 easy/medium difficulty math problems per night, he would do 3 difficult ones instead from the section.

Friend 2: He would not have to any homework for AP Physics class as long as he got an A on the previous exam.

AP Bio Teacher: Had an official rule where any student who got a 90% or higher on the previous week's exam wouldn't have to do any of the assignments for the week (though many would do the assignments anyways to learn the material).

These are some things he could look into.