1. Don't try so hard.
2. Let the teenager go off by himself. Set meet-ups for meals and for checking in. I'm sure he'll find something to do that's more interesting for him.
3. I would have suggested having him bring a friend, but that might be too late.
Well, I'm 16 and our drive to DL is about 6 hours too. I usually just bring my mp3 or cd player and sleep through most of it. We do stop about every 2-3 hours so that helps. As for Disneyland, that's usually the only family thing I don't lost interest in. There's so much to see and do I'm sure he'll have a great time!
At 15, most of those games will be an eyeroll for him. Suggestions would be for him to bring a CD or MP3 player, or a GameBoy. (Bring PLENTY of batteries.) Also, have him tote it in the park. My kids (die hard Disneylanders) always have a gameboy, CD player, and a book in their backpack at Disneyland. Makes line waits go by faster. Comes in real handy when you are sitting in front of tha Castle waiting for Remember. Since he has never been, and doesn't have a companion, I do not recommend sending him off on his own. He is old enough to hang with the adults (and it sounds like you are closer to his age anyway (early-mid 20s?)), with the aforementioned toys. Make sure he has a camera to take pics of what he wants.
If you see a cute yellow lab puppy with a yellow cape, WAVE! It might be us! (Or it may be someone else that lurks here!) Thank you for asking before you pet! Next trip, Dec 22-Jan 3rd.