He is only 5, for goodness sake! Children that age have an attention span of up to 20 minutes... or less if they are very intelligent, because they get bored quickly and want to move on to another thing! Sounds like the teacher wants an easy way out, without having to put too much effort into keeping a bright child engaged.
Maybe you could suggest to the teacher to give your DS something a bit more challenging to do and he will be fine.
This is what I've found on the website below: http://www.parents.com/kids/development ... ion-spans/
"Child development experts say that, on average, a 4- or 5-year-old child should be able to stay focused on a task for two to five minutes times the year of their age. So, young kids should be able to focus between 4 and 20 minutes, possibly more, depending on the task." How long does your DS's teacher expect he will be able to sit still and pay attention?
I've had the same problem with my eldest - throughout his primary school teachers complained he was not paying attention, suggested ADHD and even Turret's, believe it or not! Yet, he was on top of his class, got into a super-selective school and is doing very well there.
Do not let them 'statement' your child. Any form of a formal assessment, especially at such a young age, is likely to have a negative impact on your child's confidence.
I once had a chat with a very good educational psychologist, who I asked whether my DS teacher might be right and he said this: 'Teachers need to learn how to deal with a classroom full of children, or don't go into teaching. If they could, some of them would add ADHD medication to tap water'. I am not suggesting that this applies to all teachers, but sadly, you can find teachers like that in most schools.