HI, there! My son (who is 7 now) also had troubles such as this in preschool and his preschool teacher asked us to watch his breakfast foods. That was when we really started looking at ingredients in our foods. Store-bought cereals and even yogurt has a LOT of carbs! It was an eye-opener. He continued to have focus and impulsivity issues in Kindergarten so we went organic, no HFCS, no transfats, no artificial additives (food coloring, flavoring, etc.). It's easier than you would think.
We also took him to a pediatric neurologist at the end of Kindergarten who said, yes -he's ADHD but that ADHD is a symptom of many different things. He did not recommend medication until much later and only if he starts to have learning issues or behavior issues. He gave us this article for his teachers to use in school: http://www.oneaddplace.com/articles/adstud.htm
In our experience, 1st grade is much better then Kindergarten. The focus and impulsivity issues are still there, they're just a bit less of a problem this year.
Basically, the teachers and we (his parents) just have techniques to use while we keep in mind that he just may not hear what you said because his mind is elsewhere and he may not have retained the instructions you gave him.
Based on your son's specific diagnosis, he will probably outgrow much of this.