I am not sure what ages the law says, but in OK the school CAN expel a child for repeated truancy. actually if there are over either 7 or 10 days absent (excused) or 5 unexcused they take the PARENT and child to truancy court.
Being 17, I am SURE that school will consider the difficult child to have dropped out with-o telling them if there are a lot of absences. There is a certain date that federal funding is based on - $X per year per child enrolled on that date. Our schools are being scrutinized for this because right after that date there are a bunch of kids who get dropped from the rolls for "truancy".
Have you looked into online high school or an alternative school? Not all alternative schools are bad - not every kid is a good "fit" for regular high school and alternative schools can help meet their needs. Or they can be a training ground for gangs and juvie. It largely depends on the principal and the school system.
Studies have shown that teenage brains actually perform worse in many regards in the early part of the day, esp early morning. Many, if not most, teens would be well served by a high school that started at 11am or even later and went later into the day.
Would he do an online course? Many states now provide them for free for high school and for earlier grades sometimes.
If he won't do high school, would he work toward the GED? He can go to college with that.