If you cannot get the "because I said so" thing to work (it never worked on me when I was a kid), then beat his argument. I know where he is coming from. The way you described him is very similar to how I was. I was getting A's in honors classes while doing drugs and drinking... during school hours. I was so bored in school that friends used to see how much crap we could get away with. If I presented a logical argument to a authority figure and they replied with "I said so", they lost. PERIOD.
The good news, logically you are right. He should finish high school.
What is his actual argument?