Force?! You cannot! If he does not understand why, you cannot force understanding.
It is common belief that teens have to finish school in order to succeed in life, but that is not true. Sometimes, leaving school is the best thing that happens to them. For example, I was kicked out of school at the end of grade 10 and it was the best thing that happened to me. It was a strong wake-up call. In the end, I became a teacher.
Do not force him. This will just cause tension. It is just like a rubber band. The more each of you pulls, the more tension there is. With more tension, he will only do worse, because he will stop sharing his thoughts with you and asking for your advice.
Let him be! This is his journey! All you need to do is keep telling him that you are there if he needs help, to encourage him to think about what he wants to do in life and that you love him and care for him.