How can I help my middle schooler who's lost motivation to do homework?

If you suspect that your child is not completing his homework, I'd suggest initiating a conversation in a non-threatening way. For example, you might say, "I've noticed lately that you're not spending as much time as you used to doing your homework." Then allow him to offer an explanation. If the answer you receive isn't satisfactory, observe for a couple of weeks to see if there are any other changes in his behavior that may be of concern. If homework appears to be the only problem, it might be helpful to remind yourself that it's your son's responsibility to do his homework, not yours!