I am sorry you had a stressful day with your son. I truly understand.
I have learned along the way (the hard way) that his homework is his. Not mine to worry about. If he fails, he has to pay the natural consequences of his choices. (repeating the year, summer school, etc.)
I was always so stressed about my son's education. He could have cared less. Looking back, it was the least of my worries regarding his choices and behavior. Some people don't really buckle down in school until later on in life. That's OK!
My son is glad he did not have to return back to his HS either. He actually thanked me for that.
There are other options for you and your son. Maybe a tech high school to learn a computer skill? Maybe some kind of job corp program, the National guard program. There are lots of places, spend your time being resourceful and not worrying about his homework and grades. It just become a power struggle between you and him.
Reward the positive behavior and but don't enable the negative ones. He is 16, will bait you every time and feel like he won. In fact, he does win. I doubt he is as stressed as you are right now!!