I second what FlyerJosh says. If this were happening at my school, the chief instructor wouldn't stand for it unless the instructor had a valid reason for cancelling. A good flight school will not only rectify this with the instructor, but also try and make the student happy. My school has a weekly meeting with instructors to discuss student load and progress and we all try to work with each other as well as our students regarding technique and ways to present a topic. We try our best to not only work out times, but also find the best fit between student and instructor becuase we all have different personalities and as a result students will have better progress with someone they work well with.
When in doubt, remember that it's your money. By cancelling all the time he may be costing you more in the long run becuase as time lapses between lessons, what you've learned on previous lessons will become a distant memory and you'll spend more time reviewing old info on the next lesson.