Feeling more focused is a good sign. You should also be able to see that he is more focused through longer attention span, better schoolwork, positive teacher reports etc.
One day of bad mood is not significant, as that can happen for many reasons. Continued bad mood over days or weeks is significant.
Dr. will look for most effective dose without unacceptable side effects. You probably won't get 100% improvement, but if you get 70 or 80%, you are doing great.