There's not much you can do to help Germany as Japan, except join Barbarossa. And fighting the Soviets isn't in Japan's best interest in '41. That's the most frustrating thing playing as Japan; watching the Axis collapse in Europe and knowing you are on your own.
Well, you could annex the UK for them while they take out the Soviets. I mean, you got India already at this point (hopefully), so why not finish off the Allies. Doesn't that at least buy the AI some respite from bombing and whatnot?