Sometimes natural consequences are too long term. In those situations, I consider a parent's job to be converting long term consequences into short term ones. The long term natural consequence of not being educated is a diminished earning capacity. If you want the short term consequences to match, reducing the amount of money you spend on him is a good start.
However, I agree with Chrys that you need to find the underlying reasons. Try to figure out ways to make school more relevant to him, or to make up for the school's deficiencies. If he's not getting enough time to socialize with his friends, see if you can rearrange his classes, or agree to host his friends at your house as long as he attends school. If he is bored in class, get him put into more advanced classes, or teach him ways to deal with it. I used to read or do homework from other subjects when lectures became boring.
If he is struggling to keep up, see if you can transfer him to a more appropriate subject, or do some tutoring. If he is getting bullied, there are ways to deal with that. If the kinds of subjects he's interested aren't offered, or are very limited, try to find extracurricular ways to help him get experience with those topics. If he likes art, for example, get him art supplies as rewards for school attendance. I realize it's not for everyone, but my son not fitting very well with school is a major reason we homeschool him now.
Not everyone fits the school mold. Figure out how to make it worth his while.