First, you need to know whether your kid can concentrate if he wants. If yes, then the problem is only because he don't want to focus sometimes. The problem is only on some subjects, or all.
I have the same problem, but I just know from the school that my kid did not look at the teacher, even the classmate shared their view. It caused the problem on the concentration. So the advice is that I need to teach him to look at the people's eyes when they are talking. Hope the situation can be improved.