CCM,
Why would you have him call a WWASP school? That is the type of place to be avoided at all costs. Seems like a waste of the man's time to me.
The poster admitted to two mistakes my own parents made. One, the attempted use of force and, two, bribes. My Mom used to pay me to go to little league. I took the money and bought cigarettes and candy and never went to the games.
To the Dad,
Besides, if your boy is really depressed, bribes will not be enough motivation. My Dad used force which just made me better at not getting caught and drove a communication wedge between us. I was afraid to talk to him.
These are both attempts to control your son. Control over another is really an illusion as any therapist or counselor will tell you.
At the same time, don't try to punish or control him by removing positive things from him. If guitar lessons are a good thing, don't take a good thing away from him. Instead encourage the good things. Removing things he really cares about that are positive will only make him hate and resent you more.
I can't solve your problem because I don't know you or your son and there is no way anyone can diagnose your situation from a few web postings.
All I can say is that you need to build trust. Listen more, bite your tongue. Don't contradict or discredit what he says, just offer your own view point and advice. He may not appear to be listening, but he will hear you. Do this as you would to an adult friend. He will listen to suggestions more readily than orders.
Your son is approaching adulthood and you will gain a great deal of credibility with him if you treat him more like one. Also, just expect him to act like one and act surprised (but not angry) when he doesn't. Treat him like the kind of person you want him to be and see if he grows into the new image.
Ask him for advice on something, anything, a big family purchase, or a vacation decision or something that makes him feel like his view really counts. Include him on any education decisions. In other words give him a voice and a choice and the responsibility to go along with it.
You are not the boss anymore, you are the mentor, advisor, and safety net. This does not mean there are no house rules. The house rules apply to everyone, even you.
Be patient. You will not immediately stop his current behavior and the process is not linear. Success is two steps ahead, one back and cannot be measured daily. You'll just drive yourself crazy.