It sounds to me like a stage he is going through. Concerta does not "build up" in the system. If he did well for even a month without it..then he didn't really need it (of course, I am not a doctor)...It is likely just adolescence bringing out the negativity. If you re-medicated him because he started having teenage issues...and he didn't really need the medication...the medication could actually make him worse. I would look into stopping (or changing) meds BUT getting a lifecoach for him if I were you.
My oldest son is 16 and years 12-14 were ROUGH! His grades fell. He was mouthy and defiant...even tried to run away! AND he doesn't even have ADHD and never was medicated. After he got past his Freshman year..he leveled off a bit. Still has flare-ups...but a better attitude and better grades on the whole.