My daughter was on Cymbalta 6 months. She thought, like your son, that therapy just would not help her, it wasn't for her. After she went a couple times though it changed her mind. We chose a psychologist that her rheumatologist recommended, who works with lots of kids and teens with chronic illness. In her case, the psychologist and her psychiatrist said that her depression/anxiety came from her illness (she was in so much pain that year that she had to take a semester off from college and was distraught), so getting her disease under control and working with the therapist did more than Cymbalta did.
My younger daughter is more like your son - very quiet and shy, not at at all talkative. She hated talking initially and refused to see the first two psychologists because "they asked too many questions." Her GI insisted that she had to see one, which is the only reason I got her to see a third, which worked out. She's now much less stressed out and sleeps much better at night.
It's tough with teenagers because they think they know best and it's difficult (if not impossible) to "make" them go.