Well, when I was in school, I didn't have much to motivate me either. There was a time in fifth grade where, I started not turning in assignments and just leaving them in my desk.
I think that the biggest thing that you can do is show your son that you care. Another thing that would help is to have a parent-teacher conference, to also show that teachers really do care about you and want you to do the best you can. It's amazing, how showing that people care about how well you do, will motivate you.
Caring is what motivated me. I always wanted to do good, because I wanted to make my mom feel proud. It was just her and I for awhile, she didn't have much to show for life except her son, and I wanted to make her feel like she was doing something right.
I'm not trying to say that you don't care, but maybe your son doesn't see it as "caring", maybe he see's it as nagging.
Find out something that he wants, really really badly. Let him know what you really really want. See if you guys can both compromise, and get something taken care of.