L.B.
He is a bit young to have this diagnosis. His personality is still in flux and it seems a bit irresponsible on the part of the medical practitioner to make such a diagnosis.
My experience with 11 year old boys is that they go through a bit of a defiant phase at this age - sort of like an instant (but more destructive) replay of two-year-old defiance. Questioning and challenging authority, in both two-year-olds and eleven-year-olds, seems to be part of their differentiating themselves from you as a parent and part of their normal mental and emotional growth and development. Eleven year olds are more capable than two-year-olds, however, and therefore more capable of negative behavior. Because this seems to be part of a normal developmental phase at this time, I would be very, very hesitant to accept a mental illness diagnosis at this point and would be even more leery of medicating your son for it.
You need to set proper limits, treat him with respect and insist that he treat you with the same level of respect, give him as much (limited) freedom as he can handle - which may not be much, love him, love him, and love him, and realize that his behavior is not always going to be perfect. He is dealing with new hormones, new physiology, and new relationships, and does not know how to act. Above all, he needs to know that you love him unconditionally even though you might put limits on his immediate behavior.
In all likelihood, you can work him through this difficult developmental phase. Try to do it without drugs or the stigma of a mental illness diagnosis.
The bottom line: They go through the "terrible twos" about every ten years. Deal with it.
Note: My son went through this sort of phase at age eleven and turned out just fine. He is now a fine, upstanding twenty-seven year old who has a very nice personality and is not oppositional defiant in his behavior.