My younger son managed to "dump" himself out of AP English in Senior year by not handing in a long term project (that he had actually completed) in his Junior year. His teacher was ready to pull her hair out, but I told her to let him suffer the consequences. Homework was always his sticking point.
As long as the school isn't demanding your son move down a level, I'd be tempted to keep him where he is. I, too, would be more concerned with the boredom factor than actual letter grade.