Mike is my son and I’d love to tell him what it is!
Dunno, but I’ll gladly name it the Horny Poo-Bug.
Don’t you hate when your computer crashes AFTER you’ve written a brilliant and compelling post, but BEFORE you hit “Submit Reply”?
Anyway, the gist of my former but now lost post, which was laced with entertaining asides and incredibly complex logic, is that your son should submit this bug photo and description to the people who actually have expertise and who are used to identifying bugs for people:
They seem to be fairly quick about it too.
My guess: it’s a caterpillar. The legs are a dead giveaway. I just can’t figure out what kind of caterpillar. Why oh why isn’t there a site that just has tens of thousands of bug pictures, all neatly cross-referenced and collated by species, location, size, threat to mankind, etc.?
I dunno either, but you might tell him it is a bioweapon developed by the USA in response to the Japanese beetle assault they launched first. 
Hm, some interesting finds while searching:
World’s most demonic looking caterpillar?
Hello Kitty Caterpillar?
YouTube
But unfortunately after about 100 combined Google Images pages, I didn’t find it.
I’m not an expert in insects but I did find this site that may prove useful:
http://aoki2.si.gunma-u.ac.jp/youtyuu/index.html
They have a very, very extensive collection of pictures and information about all manners of Japanese caterpillars. Unfortunately, it’s all in Japanese and it uses a 90s style frame layout that’s a bit hard to navigate. Here’s the frame for “browse by image thumbnail for black, brown or grey caterpillars”:
http://aoki2.si.gunma-u.ac.jp/youtyuu/thumb-imo-d.html
That might help you get started.
(That’s not the guy you’re looking for, but I can’t help pointing out that this little guy is totally badass.)
On edit:
I think I found a good candidate: Biston robustum robstum
And another unrelated spectacular…something.
So I looked up this guy, because he amazed me. And for some reason, I can’t find any websites explaining why it looks like a snake.
I mean, I can guess why, but you think they’d mention it somewhere anyway, just because it’s so goddamned freaky!
Entomologists are weird.
I should have read my own link instead of looking at the pictures… It doesn’t look like a wilted leaf, it’s wearing a wilted leaf. Not as weird, but still cool.
One of the things that really impressed me when I first moved to Japan was the variety of mimetic insects, and how utterly convincing and detailed they are.
Anyway, some more “snakes”:
http://shoko.web.infoseek.co.jp/Guest/biroudosuzume1.html
It’s an inchworm, so a caterpillar of a geometrid moth.
Beyond that it’s hard to tell from such a low resolution photograph. The triangular head is a result of “horns”. These horned forms are fairly common throughout SE so it doesn’t surprise me that they’re found in Japan as well.
Sorry, mod, got this in the wrong place. Please delete. Thanks
I think you missed… 
ETA: and my timing sucks… lol
S^G
Thanks all!