My kiddo is about the same age - 13, and he has gone from a size M boys to a size M men's this school year (we had to buy 3 pairs of dress pants and shoes for three concerts - oh joy). He's definitely clumsier and has hit the awkward, gangly stage where he knocks stuff off counters and bangs into things because he just doesn't have his space figured out yet.
But his output has actually gotten better - a lot better. He is finishing homework fast enough to start being disruptive in class by talking to his classmates and has started the whole I'm bored routine - both of which didn't happen before this year. It wasn't that he wasn't disruptive before now - it was just for different reasons. In elementary it was because he was avoiding things he couldn't do. It's been kind of refreshing to have the whole "find something to do so you don't get you or your classmates in trouble" talk with him.
I'd second the checking into his blood sugar or his vision or something else.
