Composition: If this weren't a candid portrait, I would give you hell over the far eye being almost lost. LOL! But, since it is a candid, I say you caught that teenager expression just right :)
Lighting/Exposure: Your light is beautiful. I am guessing a window in front of him? :)
Pose: N/A
Suggestions for Improvements: Removal of the lights in the BG is really the only thing I can see here.
Candid portraits really are harder than studio posed portraits because you can rarely control the environment. You are a brave soul :)
hahah, ah the teenager years, what fun. he does look a bit like, "do this again, and i'lll go beserk." sort of look, but it makes for a good photo. composition is very well done, and the light oof adds to the homely sense of the photo. good job.