My son is 11 and while he has good grades and isn't disrespectful, his level of concentration has gone down a bit. I believe this in part to be due to hormones and a growing spurt he is going through at this time (as he is definitely getting taller) ... some might think 11 is too young for the hormone effect, but not me.. It has played a role in my son's behavior..
Also, have you considered your son's diet. I don't know if it's heavy on the sugar and processed foods.. but you may want to keep a food journal and notice if there are changes in his behavior after eating specific foods..
As someone who is currently abstinent from sugar at this time , I can tell you that I feel a HUGE difference.. I have had low grade depression from time to time and since I have been off the sugar, I have been a great deal better.. Mind you, I didn't really even eat too much of it, so goes to show that it may not even take much sugar to affect one's brain chemistry..
I like to take a natural approach .. try a new diet and see if this helps.. Also, maybe sit down and talk with him.. preferably at night when he might be more relaxed and open to it.. maybe right before bedtime... check in with him and ask him about his day... He must might open up and tell you what's on his mind..
good luck