Are they likely to ask about detentions when they ask the school for a reference?
I've been told that the format of references requested can vary from school to school. Assuming e.g. Eton might request information regarding detentions, I imagine they would also ask for details of the reason for such a detention, and 'over-enthusiastic / too chatty in class' does not sound like a very serious misbehaviour! They know what 10/11 year old boys are like and if your DS is very academic and scores highly in the exams, I don't think he will have any problems. If you're worried, though, you may want to tell him to keep a bit lower profile until the exams are over.