Hi, M.. Hon, I wouldn't worry about ADHD just yet. It sounds like your son is #1, bored, and #2, has the typical attention span of a 5-year-old boy. If he had ADHD, he would probably have much worse impulse control and would probably be at least a little bit of a behavior problem.
A lot of kids with supposed behavior problems or attention problems are simply kids who are bored and are not having their learning styles accomodated.....
It could be an immaturity problem on top of that as well. He doesn't have a sense of duty or responsibility to finish the work. If he has absolutely no problem with the work itself, then he's probably bored at doing the same things over and over again. My son was like that, too, learned things very quickly and then wanted a challenge instead of repeating the same things over and over again.
Here's what I would suggest. #1, instead of giving him a full sheet of work to do, the teacher should draw a line across the page and tell him to do up to the line, then he can get a reward or switch to something else. Then, later on or the next day, or whenever it's convenient, he can finish the rest of the page. That way, his short attention span (again, no need to call him ADHD yet) will not have to be taxed, and he can feel good about what he does accomplish instead of always being told he's wrong for not finishing.
I, personally, think that it's crazy how pre-schoolers and kindergarteners are expected to sit and concentrate the way older children are expected to. These kids aren't old enough for that! Some of them can cope with such a boring environment, but it's not the right environment for everyone. Having the wrong expectations of a 5-year-old poisons him against school altogether and makes him less willing to try anything or to compromise his needs for the sake of the class -- since everything about the class feels bad because he's bored!
One thing I would suggest is finding a way to have him evaluated for his learning style strengths. If he's not a visual learner, then he's going to need to rely more on his hearing or physical, tactile senses in order to be successful.
For instance, if he's permitted to say the questions out loud while he's doing the work, then he might enjoy it more than relying only on his sight, which is not the most interesting sense to him. Or if he's allowed to mold things with clay -- for instance, instead of always having to write the letter "C" ten times, maybe he can write it 5 times and make a "C" out of clay, or cut one out of paper; this will satisfy his tactile or movement needs while teaching him the alphabet.
I'll be the other kids would learn better, too, if more of their senses were stimulated instead of feeling chained to a desk with a pencil and boring piece of white paper!
Ask his teacher about learning style theory, and why is it that she doesn't try to accomodate all the senses of all her students. See what her answer is; these days, all education majors in college have to learn about learning styles, and all of them are supposed to incorporate such things in their classrooms daily.
Please keep us posted on how he's doing. I will say a prayer for a better situation for him in the classroom.
Peace,
Syl