I would tell him that there are many, many people working very hard to make sure everyone will have a place to live and to rebuild their towns. But these are grown up jobs and responsibilities and grown-ups will take care of it. If he likes, your family can make a donation to help the people of Japan.
I would not share any more than that, and in fact, I would cut him off about it after a few minutes of discussion. Talking too much about something often serves only to reinforce their fear.
When he brings it up again (and he will), I would say something like, "Remember how we said that the grown-ups were helping? Well, they are still working hard. And remember, this is something for grown-ups to worry about."
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)