How much would my son have to invest per year at 6% to have 1 million dollars after 20 years? How about at 9%? At 12%? Is there any easy way I can help him figure these amounts out on his own?

Really depends on the currency.
The current echange rate in Zimbabwe is 250 revalued Zim dollars per 1 US dollar, so wouldn't take long at all ;)

Or how about Japanese Yen:
1 US$ = 119.35 Japanese yen

How much would my son have to invest per year at 6% to have 1 million dollars after 20 years? How about at 9%? At 12%? Is there any easy way I can help him figure these amounts out on his own?
At 6%, $27,184.56,
At 9%, $19,546.48,
At 12%, $13,878.78.
Get him a financial calculator.