I have a 10-yr-old son. He's a fifth grader right now and I have been really worried about his attitude toward homework and everything that has to do with studying. He constantly calls himself stupid and has no motivation for getting good grades. He has many friends, so I have no worries in that aspect. He is half American, half Japanese. He goes to a public Japanese school. At first, I thought maybe it was because of his inability to understand the Japanese language as well as other children his age since we were living in the States until he was 3 and a half. But, in my opinion, him being 10, it's been long enough to make up for that time now. I try to come up with things that might motivate him, but it doesn't seem to work. People have told me that around the time when he's 12, he will be able to cover everything that's been taught in elementary school, but I'm not completely buying it. How can I motivate him?
Not knowing your son I can only speculate, but the whole description sorta hits home with me, so here's my stab into the dark and advice:
He's probably incredibly bored by school. I know second hand that Japanese schools don't have particularly inventive teaching methods, and he's probably bored by sitting and listening all day to a teacher who may not be very interesting talk about topics he doesn't have any particular interest in. It can bore the crap out of anyone, and nobody wants to have anything to do with boring topics when there are alternatives (friends and games).
Fundamentally the things being taught in school are fascinating, the problem is that the presentation can often obscure that fact and that often no basis is being taught for why the subject is actually quite interesting. Also the fact that all subjects taught in school are fundamentally interconnected is often missed in my experience. So your son may be wondering what he's supposed to do with this useless knowledge he's supposed to retain and/or why it might be interesting to anyone at all.
To help him there, you can try to fill in the gaps between the theoretical world of school, practical daily life and the real world. Should he suddenly develop an interest in astronomy for example, he'll gobble up physics, maths and other related subjects automatically, because he can suddenly see how they help him understand a topic he cares about. Or an interest in war machinery will feed an interest in history. And on and on it goes. Help him find something to connect the dots. A new hobby could be it, but perhaps he also needs more exposure to the real world in the form of travel, a job, practical responsibilities or something of that sort. Whatever is practically available to you and sticks with him.