i think if anyone suffers from any depression disorders its up to the parent to force child even if child is older to go to a psychiatrist it will make both u and child feel better id say bringing child to a professional is an obligation. on a different note try to select a trusted psychiatrist who your child can unload on.

Therapy will tend to not be effective if there against one's will.  Learn positive persuasion and influence and what motivates him.  
Discussion and questions will help, but a therapist knows how to ask those questions so the cycle continues.  A good therapist could give you guidance on this as that is where I often start with getting the job done!
Edie

On second thought, I would first try with influence and persuasion and I can do that, but if that is not your core competency and the child is seriously depressed, you do have to take action.  I just like to get it done WITH them, but you may have no choice but to say, I LOVE YOU and thus I need to get you help.   Do it with love so the child doesn't feel you are an adversary.  
Hope this helps.