Looking for suggestions

I have a great class. Truly great. I love the mix of personalities, the creativity, and the different learning styles. 
But...
I have this one kid, just one, who drives me nuts. He is a good kid. Really. And I can see that he is smart. Unfortunately, he refuses to complete ANY class work. It's not an aggressive refusal, it's a lazy refusal. So, all that "smart" that I see can't always be assessed. Plus, he missed the entire first quarter (because his family was travelling) so I have a feeling he doesn't see his education as important. I've talked with him. I've talked with his Mom. I've talked with his older sister. I've talked with our principal. I've talked with our vice-principal. He's talked with the principal even! I hear from his family that this is the same behaviour he showed in his last school.

So I made an incentive chart to track {what little} work is actually completed. He was made fully aware of the rewards and consequences and even agreed it would help. I promised rewards like positive phone call home! The treasure box! The chance to change seats at snack and lunch! It didn't work.

I researched. I read all of this information. I gave him a dependable buddy in class. I gave him a little extra of my time to explain assignments. I called on him in group discussion and he was proud to answer! {Hurray for small successes!} I talked seriously with him about the importance of school, his goals, and of course I was very firm and consistent with consequences. I helped him start assignments. I praised and praised and praised and still do, as often as I can. It didn't work.

I made a behaviour contract, went through it with him, saw a bright glimmer of motivation in his eyes and smile; I almost screamed with joy at my hopefulness! But in the very next class - the very next class, I say! - the kid still would not do a thing. And when I say he won't do a thing, I really do mean that - he won't write his name. He won't even colour. He barely participates. I feel like I'm not sure what to do anymore.
Any help? How do you motivate the unmotivated?