H.W.
First, I agree wholeheartedly that the language around being 'smart' needs to change. Here's an excellent article in the New York Magazine by Po Bronson which I really think speaks to the problem with the "smart" label:
http://nymag.com/news/features/27840/
You've received a lot of good advice, so I will add in one more thing-- I think your son, at 15, needs to be present for this meeting and every single meeting regarding his education. He's old enough to hear the truth-- if he's failing, HE needs to see how that failure affect his academic future. You being the only person having to deal with the emotional work of these meetings is nonsense. This isn't a punishment, this is a natural consequence of his not doing the work.
A lot of people suggest having your son find his passion. While that is a great idea, it may be that he chooses NOT to be interested/motivated, and then, surviving is really not about getting to do something you love, it's about cultivating basic skills like task follow-through and giving a care enough to keep one's job. Do talk to a mental health counselor and have him checked for depression, because I think that's important, but you need to also make sure he understands that your expectations include him being able to either go to college or work after he has graduated.